Gamma-ray Burst 990123

A bright (peak count rate about 33400 cts/s) Gamma-Ray Burst (BATSE trigger No. 7343) was detected with BATSE, COMPTEL and BeppoSAX on January 1, 1999 at 09:46:56 (BATSE) and 9:47:14 (BeppoSAX) UT. Localization with the BeppoSAX WFC allowed follow-up observations with the NFI starting about 6 hrs after the GRB.
(all information courtesy of the instrument teams)

Previous IAU Circulars

IAUC 7094 suggested optical counterpart at R=18.2 mag
IAUC 7095 WFC detection, position and intensity of GRB
IAUC 7096 optical brightness and colors; redshift of OT
IAUC 7098 further photometry; photometric accuracy of USNO A2.0
IAUC 7099 WFC NFI observation of fading X-ray afterglow; photometry
IAUC 7100 ROTSE detection of OT

Results of Observations

BATSE alert

TITLE:         BACODINE FINAL BURST POSITION NOTICE
NOTICE_DATE:   Sat 23 Jan 99 09:49:27 UT
NOTICE_TYPE:   Final
TRIGGER_NUM:   7343
GRB_RA:        237.06d {+15h 48m 14s} (J2000),
               237.05d {+15h 48m 12s} (current),
               236.71d {+15h 46m 50s} (1950)
GRB_DEC:       +51.53d {+51d 31' 38"} (J2000),
               +51.53d {+51d 31' 48"} (current),
               +51.68d {+51d 40' 45"} (1950)
GRB_ERROR:     0.2 [deg radius, statistical only]
GRB_FLUENCE:   280829 [cnts]    Peak=33374 [cnts/sec]  Del_t=27.6 [sec]
GRB_TIME:      35216.12 SOD {09:46:56.12} UT
GRB_DATE:      11201 TJD;    23 DOY;   99/01/23
GRB_SC_AZ:      81.75 [deg]                        {XScan=62.43}
GRB_SC_EL:      26.40 [deg]  {Zenith_angle=63.60}  {Scan=16.12}
SC_X_RA:       282.14 [deg] (J2000)
SC_X_DEC:      -21.06 [deg]
SC_Z_RA:       192.24 [deg]
SC_Z_DEC:        0.25 [deg]
SUN_POSTN:     305.23d {+20h 20m 55s}  -19.50d {-19d 30' 04"}
SUN_DIST:       92.49 [deg]
MOON_POSTN:     15.16d {+01h 00m 39s}   +1.93d {+01d 55' 53"}
MOON_DIST:     115.88 [deg]
PROG_VERSION:  5.23
PROG_LEVEL:    1
COMMENTS:      Definite GRB.  
COMMENTS:      This notice is based on 32 1.024-s samples of the light curve. 
COMMENTS:      In COMPTEL FOV. Possible COMPTEL location to follow.

BATSE lightcurve
OSSE lightcurve

COMPTEL localization

NOTICE_HEAD: COMPTEL BURST POSITION NOTICE: GRB990123
NOTICE_TYPE: INITIAL
NOTICE_DATE: Sat Jan 23 09:57:56 GMT 1999
ORIG_SENDER: burst@comptel.unh.edu
RBR_VERSION: SRCRBR version 13.0-COMPASS
*****************************************************
BURST_DATE : 11201 TJD; 23-Jan-1999
BURST_TIME : 35216.12 sec; 09:46:56.12 UT
-----------------------------------------------------
BURST_SCZEN:  53.81 deg     {OScan=   4.05}
BURST_SCAZI: 272.97 deg     {XScan=  53.70}
BURST_CLASS: MEDIUM  (Approx   51 events)
BURST_DETEC: MEDIUM  (Approx  8.2 sigma)
*****************************************************
COORD_J2000: [RAsc       Decl] [RAsc            Decl]
============ ================= ======================
MAXPOS_J20 : [221.47d  47.67d] [14h45m53s  47d40'22"]
-----------------------------------------------------
ERRBOX1_J20: [220.66d  48.88d] [14h42m39s  48d52'39"]
ERRBOX2_J20: [219.41d  48.09d] [14h37m38s  48d05'15"]
ERRBOX3_J20: [224.92d  43.80d] [14h59m42s  43d47'60"]
ERRBOX4_J20: [226.18d  44.55d] [15h04m44s  44d32'45"]
*****************************************************
COORD_B1950: [RAsc       Decl] [RAsc            Decl]
============ ================= ======================
MAXPOS_B50 : [221.07d  47.89d] [14h44m18s  47d53'26"]
-----------------------------------------------------
ERRBOX1_B50: [220.27d  49.10d] [14h41m05s  49d05'53"]
ERRBOX2_B50: [219.00d  48.31d] [14h36m01s  48d18'43"]
ERRBOX3_B50: [224.51d  44.01d] [14h58m03s  44d00'21"]
ERRBOX4_B50: [225.78d  44.75d] [15h03m08s  44d44'50"]
-----------------------------------------------------
NOTE01-----: Error box indicates 2-sigma statistical
NOTE02-----: confidence limits only.
NOTE03-----:          -----
NOTE04-----: BATSE Trigger No. 007343

COMPTEL update

NOTICE_HEAD: COMPTEL BURST POSITION NOTICE: GRB990123
NOTICE_TYPE: UPDATE1 (includes error contour points)
NOTICE_DATE: Sat Jan 23 10:01:33 GMT 1999
ORIG_SENDER: burst@comptel.unh.edu
RBR_VERSION: SRCRBR version 13.0-COMPASS
*****************************************************
]
*****************************************************
COORD_B1950: [RAsc       Decl] [RAsc            Decl]
============ ================= ======================
MAXPOS_B50 : [221.20d  48.07d] [14h44m49s  48d04'06"]
-----------------------------------------------------
ERRBOX1_B50: [219.69d  49.41d] [14h38m45s  49d24'45"]
ERRBOX2_B50: [218.82d  48.63d] [14h35m16s  48d37'47"]
ERRBOX3_B50: [221.40d  47.33d] [14h45m37s  47d19'58"]
ERRBOX4_B50: [222.29d  48.10d] [14h49m10s  48d05'45"]
-----------------------------------------------------
NOTE01-----: Error box indicates 2-sigma statistical
NOTE02-----: confidence limits only.
NOTE03-----:          -----
NOTE04-----: BATSE Trigger No. 007343
NOTE05-----: Please DO NOT reply to this message!
NOTE06-----: Questions? Contact COMPTEL duty scientists
NOTE07-----: A Connors, A Young, S Kappadath, J Ryan
NOTE08-----: at burupdate@comptel.sr.unh.edu
*****************************************************
Burst location uncertainty contour (2-sigma) follows.
 50 contour points, J2000 coordinates in deg
     IP    RAsc    Decl      IP    RAsc    Decl
-----------------------------------------------------
     01   222.17   48.15     26   221.47   47.67
     02   219.73   48.87     27   221.47   47.67
     03   221.80   47.12     28   221.47   47.67
     04   221.47   47.67     29   221.47   47.67
     05   221.47   47.67     30   221.47   47.67
     06   221.47   47.67     31   221.47   47.67
     07   221.47   47.67     32   221.47   47.67
     08   221.47   47.67     33   221.47   47.67
     09   221.47   47.67     34   221.47   47.67
     10   221.47   47.67     35   221.47   47.67
     11   221.47   47.67     36   221.47   47.67
     12   221.47   47.67     37   221.47   47.67
     13   221.47   47.67     38   221.47   47.67
     14   221.47   47.67     39   221.47   47.67
     15   221.47   47.67     40   221.47   47.67
     16   221.47   47.67     41   221.47   47.67
     17   221.47   47.67     42   221.47   47.67
     18   221.47   47.67     43   221.47   47.67
     19   221.47   47.67     44   221.47   47.67
     20   221.47   47.67     45   221.47   47.67
     21   221.47   47.67     46   221.47   47.67
     22   221.47   47.67     47   221.47   47.67
     23   221.47   47.67     48   221.47   47.67
     24   221.47   47.67     49   221.47   47.67
     25   221.47   47.67     50   221.47   47.67
-----------------------------------------------------

BeppoSAX MAIL n. 99/1 = GCN notice #199
On Jan.23, 9:47:14 UT BeppoSAX GRBM has been triggered by a burst, GB990123 (also BATSE trigger n.7343), the strongest detected so far simultaneously with the WFC.
Preliminary coordinates from WFC are:
```
R.A.(2000)= 231.374
DEC(2000)= +44.754
```
with an error radius of about 5'
A follow-on with NFI is being planned.
GCN notice #200
D. A. Frail (NRAO), and S. R. Kulkarni (Caltech) report on behalf of a larger NRAO/Caltech collaboration:
"We observed the error box of GRB 990123 (GCN #199) beginning January 23.63 UT with the Very Large Array (VLA) at 8.46 GHz. There are no radio sources brighter than 170 microJy (5-sigma) in the WFC error circle. Further observations are planned."
GCN notice 201
S. C. Odewahn, J. S. Bloom, and S. R. Kulkarni (CIT) report on behalf of the Caltech-NRAO-CARA GRB Collaboration:
"We imaged the BeppoSAX localization (Piro et al. 1999; GCN #199) of GRB 990123 with a CCD camera on the Palomar 60-inch. In each 400-s R-band image a source clearly brighter than on DSS was seen at position: RA,DEC = 15:25:30.53, +44:46:00.5 (J2000) (preliminary astrometry). This localization is well-within the BeppoSAX error radius. Though DSS and R-band are somewhat different bandpasses, given the apparent brightness of the source relative to the DSS image, we suggest that this source is the optical transient of GRB 990123.
A faint source at the limit of detectability is seen on the POSS-II image at this location and we suggest this may is the host galaxy of the GRB. If true, we have discovered the brightest host galaxy of any GRB known thus far --- this would be commensurate with the expectation, based on the high GRB flux as measured by BeppoSAX, that this burst is nearer than any localized previously.
Preliminary photometry based on the APM catalogue puts the optical transient at R = 18.2 (Jan 23.565 UT) and the host galaxy at R ~= 21.3. More precise photometric and astrometric calibrations are currently under way.
The discovery image may be obtained at the following web page: http://astro.caltech.edu/~jsb/GRB/grb990123.html and is reproduced below:
Follow-up of this most bright burst is urged at all wavelengths.
BeppoSAX MAIL n. 99/2 = GCN notice #202
Refined coordinates from WFC are:
```
R.A.(2000)= 231.369
DEC(2000)= +44.758
```
i.e. 0.3' away from the preliminary position.
The error radius is 2'.
The follow-on with NFI should start around 15:40 UT, i.e. less than 6 hr after the burst.
BeppoSAX MAIL n. 99/3 = GCN notice #203
A BeppoSAX follow-up of GRB990123 initiated around 15:40 UT, i.e. 6 hr after the burst
Preliminary analysis of the first 20 minutes of the MECS data at SOC shows a previously unknown strong source (about 10**-11 c.g.s in the 1.6-10 keV) in the center of the WFC error circle.
Preliminary coordinates are:
```
R.A.(2000)= 231.374
DEC(2000)= +44.758
```
The error radius is 1.5'
Considering its strenght, this X-ray source is very likely the X-ray afterglow of GB990123 We will continue to observe it to monitor its temporal evolution.
GCN notice #204
J. Zhu, H. T. Zhang, on behave of the Beijing Astronomical Observatory GRB team, report:
"R-band image of the BeppoSAX WFC error circle of GRB 990123 (Piro, GCN #199) were obtained on 1999 Jan 23.756 UT, 8.5 hours after the GRB, with the BAO 0.6/0.9m Schmidt telescope in Xinglong. The weather was bad, so only one 20-min. exposure image taken under thin cloud was usable (FWHM=6").
Central part of the image is posted at http://vega.bac.pku.edu.cn/~zj/grb/grb990123.gif.
A faint object could be seen closed to the optical candidate position suggested by S. C. Odewahn et al. (GCN, #201). Its position from our measurement is RA=15:25:30.28, Dec=+44:45:59.0 (1 sigma = 0.5"), with magnitude of 19.2 (+/- 0.5 ?) if using the following 4 stars' magnitude information from the USNO-A V1.0 catalogue (David Monet, et. al.):
```
=========================================================
No. RA_mea (2000.0) Dec_mea  RA_cat (2000.0) Dec_cat mag.
--- -----------------------  ----------------------- ----
1   15:25:27.03 +44:46:23.3  15:25:27.04 +44:46:23.2 14.4
2   15:25:36.47 +44:44:37.6  15:25:36.45 +44:44:37.6 15.3
3   15:25:32.57 +44:44:29.9  15:25:32.66 +44:44:29.9 18.5
4   15:25:27.42 +44:44:42.5  15:25:27.48 +44:44:43.6 19.7
=========================================================
```
The object in our image seems slightly extended and slightly southwest comparing with the discovery image of Odewahn et al., but it seems to be impossible to confirm from only one image in not good quality. However, it could be concluded that if such an object (in our image) would not be real, the optical transient in Odewahn et al.'s images must be faded largely between the 4.6 hours interval.
GCN notice #205
C. W. Akerlof and T. A. McKay (Univ. of Michigan) report on behalf of the ROTSE collaboration (Michigan/LANL/LLNL):
We observed the error box of GRB 990123 provided by the BACODINE Burst Position Notice dated 23-Jan-99 09:46:59 using the ROTSE-I telephoto camera array located at Los Alamos, New Mexico. The first exposure began at 9:47:18.30, 22.18 seconds after the nominal burst trigger time. A rapidly fading object was discovered at the coordinates, RA = 231.3754, DEC = 44.7666 (J2000) which is within 1/3 of a pixel of the optical counterpart reported by Odewahn et al. (GCN #201). The light curve for this object is relatively complex: the luminosity increases by 3 magnitudes between the first and second exposures. Estimated magnitudes for the first six exposures are given below:
```
        UTC     exposure     m_v

    9:47:18.3    5 secs.    11.82
    9:47:43.5    5 secs.     8.95
    9:47:08.8    5 secs.    10.08
    9:51:37.5   75 secs.    13.22
    9:54:22.8   75 secs.    14.00
    9:57:08.1   75 secs.    14.53
```
Note that the ROTSE-I detector system uses an unfiltered broadband CCD so that magnitude estimates are based on comparisons to catalog values for nearby stars. Sky patrol images of the same coordinates taken 133 minutes earlier showed no evidence of the transient to a limit of at least two magnitudes deeper. A more extensive analysis of this data will be available in the near future.
The discovery images will be posted on the ROTSE Web page at: http://www.umich.edu/~rotse/gifs/grb990123/990123.gif and is shown below:
GCN notice #206
J. S. Bloom, R. R. Gal, L. L. Lubin, J. Mulchaey, S. C. Odewahn (CIT), S. R. Kulkarni report on behalf of the Caltech-NRAO-CARA GRB Collaboration:
"We obtained a single B-band image (300-s) at the Palomar 200-inch of the field of GRB 990123 localized by BeppoSAX (GCN #200). Using images obtained of the Landolt standard field 98, we derived a photometric zeropoint for the GRB data. The transient first reported in Odewahn, Bloom, Kulkarni (GCN #202) is well-detected at B = 18.93 +/- 0.03 mag (Jan 23.578 UT). The uncertainty includes the systematic and statistical uncertainties though no color term was applied. For reference, we find the following B = 19.59 +/- 0.04 for the object at position ra: 15:25:32.7, dec: +44:44:29.7 (J2000).
Absolute astrometry was obtained by comparison of 34 objects near the optical transient with the USNO-A2.0 Catalogue. The r.m.s. uncertainties of the astrometry are 0.28 arcsec (ra) and 0.26 arcsec (dec). We find the position of the optical transient to be, ra: 15:25:30.34, dec: +44:45:59.1 (J2000). Based on the POSS-II F-Plate of this field, the purported host galaxy (see GCN #202) appears offset from the OT by 1.8 arcsec (+/- 0.4 arcsec) to the South. Further analysis is underway.
GCN notice #207
R. R. Gal, S. C. Odewahn, J. S. Bloom, S. R. Kulkarni (CIT), D. A. Frail (NRAO) report on behalf of the Caltech-NRAO-CARA GRB Collaboration:
"We report on a detailed analysis of images obtained at the Palomar 60-inch telescope, the initial results of which were reported in GCN 201. Three images of 400-s duration each were obtained of the field of GRB 990123 localized by BeppoSAX (GCN #200). These images were obtained in the Gunn r-band and not R band as reported in GCN #201. The seeing was 1.7 arcsec (FWHM) in each image. Using images obtained of two Gunn standards (Feige 34 and Feige 67), we obtained a photometric zero point for the data. The observation times and magnitudes for the optical transient reported in Odewahn, Bloom, Kulkarni (GCN #202) and the reference star from Bloom et al. (GCN #206) are:
```
                                 Gunn-r Mag
UT (23 Jan 1999)            OT             Ref. Star
---------------------------------------------------------
13:37:20.3           18.70 +/- 0.04      19.10 +/- 0.04
13:51:03.6           18.78 +/- 0.04      19.11 +/- 0.04
14:02:56.5           18.75 +/- 0.06      19.07 +/- 0.06
--------------------------------------------------------
```
The uncertainty in the magnitudes include the systematic and statistical uncertainties, although no color term was applied. We note that the second r-band image was taken nearly simultaneous with the B-band image discussed in Bloom et al. (GCN #206).
The Galactic extinction in the direction of the optical transient (l,b = 73.12, 54.64) is E(B-V) = 0.01597 (Schlegel, Finkbeiner, and Davis; 1998 ApJ, 500, 525). Thus (assuming Rv = 3.1), A_B = 0.069 and A_gunnr = 0.041. The extinction corrected magnitudes of the transient at epoch Jan 23.578 1999 UT are thus B = 18.86 and Gunn-r = 18.74. Assuming the bandpass zeropoints and effective central wavelength of the Johnson B and Gunn-r bandpasses from Fukugita, Shimasaku, and Ichikawa (1995 PASP, 107, 945) we get F(B)= 115 microJy F(r) = 94 microJy. Thus the power law slope beta = 0.5 where beta = log F(nu/log(nu). the transient is blue with a spectral index of beta = 0.5 (with beta =dlog F(nu)/dlog nu). This spectral behavior has not been seen in previous GRB afterglows. In the framework of the afterglow models, this result can be interpreted to mean that the flux from the afterglow did not peak in the optical bands by the epoch of our second r-band image.
GCN notice #208
J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, S. C. Odewahn, R. R. Gal, L. M. Lubin (CIT), and D. A. Frail (NRAO) note on behalf of the Caltech-CARA-NRAO GRB collaboration:
"After an initial rise to about 1 Jy, the early-time optical light-curve of the transient afterglow of GRB 990123 (as reported by the ROTSE Team; GCN #205) displayed a power-law decay. We note here that the decay appears to be well-continued by the later time (~3 hours) optical fluxes of the transient discovered at Palomar (GCN #201; GCN #206; GCN #207). The decay constant is approximately alpha = -1.5 (defined as F_nu (t) = const * t^alpha). This is consistent with the decay constants measured at late-times in other GRB afterglows.
This suggests a strong phenomenological connection between the emission mechanism(s) at very early times (from tens of seconds to few minutes after the burst) and at later times (>~ few hours). The continuity of the power-law light curve represents a strong constraint for theoretical models of optical emission from GRBs and their afterglows.
A lightcurve, which will be actively updated, may be obtained at http://astro.caltech.edu/~jsb/GRB/grb990123.html and is reproduced below (from Jan. 29):
GCN notice #209
V. Sokolov, S. Zharikov (SAO-RAS) and L. Nicastro, M. Feroci, E. Palazzi on behalf of the BeppoSAX team report:
We obtained an R-band image of the BeppoSAX localization of GRB990123 on 1999 Jan 24.076 UT, 16.04 hours after the GRB event, with the SAO-RAS 6-m telescope. The weather was cloudy. In spite of that, in a 15-min exposure image we were able to see an object at the position of the optical candidate suggested by Odewahn et al. (GCN #201). We estimated a magnitude of 19.6 +/- 0.2 (using star 1 from GCN #204) and 19.9 +/- 0.2 (using star 2 from GCN #204). These values include the contribution of the proposed underlying host galaxy. To have the contribution of the OT alone these value should be increased by about 0.25 magnitudes.
R = 19.9 +/- 0.2 is compatible with the power-law decay with index -1.5 found by Bloom et al. (GCN #208) as we would expect to find about R = 20.1 using the R = 18.2 on Jan 23.565 UT and R = 21.3 for the underlying galaxy (GCN #201).
GCN notice #210
Eran Ofek and Elia M. Leibowitz, Wise Observatory, Tel Aviv University report:
From 4 CCD frames of the object taken at the WO between Jan 24.0617-24.1162 we have determined an average R magnitude of 19.87+-0.2, with calibration based on 4 USNO A2.0 stars. The decay parameter in the R band over that time interval is -1.9+-0.1. On Jan 24.0625, the color index of the object was B-R=-0.48+-0.25
GCN notice #211
D. A. Frail (NRAO), and S. R. Kulkarni (Caltech) report on behalf of a larger NRAO/Caltech collaboration:
We obtained a second VLA observation of the error box of GRB 990123 (GCN #199) beginning on January 24.65 UT at 8.46 GHz. At the location of the optical transient (GCN #201) there is a new radio source with a flux density of 260 +/- 32 microJy. A similar image taken on January 23.63 UT (GCN# 200) gives a 2-sigma upper limit at this same location of 64 microJy. We are likely observing the slow "turn-on" of the afterglow. Further observations are planned to track its expected rise to maximum and its subsequent power-law decay."
GCN notice #212
T.J. Galama, P. Vreeswijk, E. Rol (U. of Amsterdam), R. Strom (NFRA and U. of Amsterdam), J. van Paradijs (U. of Amsterdam and U. of Alabama in Huntsville), C. Kouveliotou (USRA/MSFC), G. de Bruyn (NFRA and U. of Groningen) report:
We observed the error box of GRB 990123 (GCN #199) at 4.88 GHz with the Westerbork Synthesis Radio Telescope (WSRT) for 12 hours on January 24.28 1999 UT. At the position of the optical transient (GCN #201) we do not detect a radio source (< 130 microJansky; 3 sigma). This result, when compared with the January 24.65 UT 8.46 GHz VLA detection (260 +/- 32 microJansky; GCN #211) may be due to synchrotron self-absorption, or otherwise, may indicate that the 4.88 GHz observation was suppressed by interstellar scintillation. Further observations are planned.
GCN notice #213
R. R. Gal, S. G. Djorgovski, S. C. Odewahn, J. S. Bloom, and S. R. Kulkarni (CIT), on behalf of the Caltech DPOSS team and the Caltech-CARA-NRAO GRB collaboration report:
A faint galaxy coincident with the optical transient discovered at Palomar (Odewahn et al., GCN Circ. 201 and IAUC 7094) was detected on the F (red) plate of Digital POSS-II (DPOSS), taken on 05 July 1994 UT. The galaxy was not detected on J (blue/green) and N (near-IR) plates taken in March 1993.
Using the new CCD calibration of the field, we obtain for the galaxy the Gunn magnitude r = 21.9 +- 0.4 (corresponding to Johnson R = 21.5 +- 0.5). Our preliminary calibration gives the upper limits in the other two band, with g > 22.4 (roughly, B > 22.0), and i > 20.6 (roughly, I > 19.9), with a net uncertainty of about 0.5 mag. The limits in g and i are consistent with the plate limits and the weak detection in the r band, for typical galaxies at these magnitude levels.
At this r band magnitude level, the typical redshift for a normal field galaxy is about 0.2 to 0.3. It is extremely unlikely that this galaxy is at z > 0.5, unless it contains an active nucleus. Future spectroscopic observations will establish if this is indeed the host galaxy of GRB 990123.
The DPOSS image can be seen at: http://astro.caltech.edu/~george/grb/grb990123.html
GCN notice #214
E. Falco, C. Petry, C. Impey, A. Koekemoer, and J. Rhoads report on behalf of the KPNO GRB Followup team:
Falco, Petry, and Impey have observed the optical counterpart of GRB 990123 in U band on 1999 January 24.497 UT using the 4 meter Mayall telescope and CCD Mosaic Camera at Kitt Peak National Observatory. Conditions were non-photometric with 1.3 arcsecond seeing.
The counterpart (cf. Odewahn et al, GCN 201) is clearly detected. Absolute flux calibration is not yet possible, but the transient is approximately 1.1 magnitude fainter than the reference object at position RA=15:25:32.7, dec=+44:44:29.7 (J2000). The statistical uncertainty in the flux should be of order 3%. The counterpart is a point source in our data, suggesting that host galaxy flux contributes only a small fraction of the current U band brightness. We suggest that the afterglow may be bluer than the host galaxy; if so, U band would be a good wavelength to follow its light curve to late stages.
In addition, Koekemoer has observed the counterpart at K band on 1999 January 24.546 UT using the 2.1 meter telescope and ONIS near-infrared camera at Kitt Peak National Observatory. Total exposure time was 600 seconds. The transient is not readily apparent in the coadded image. The rough limiting magnitude of the K band image can be estimated from the USNO-A1 catalog, which gives Red and Blue magnitudes 14.4 and 15.5 for a star with signal to noise ratio 10 in the combined K band image. More careful processing might improve this limit modestly. Combining these two measurements will yield a limit on the spectral slope of the afterglow.
A section of the U band image is available at http://www.noao.edu/noao/grb/990123.html .
GCN notice #215
P. Garnavich, S. Jha, K. Stanek and M. Garcia (Center for Astrophysics) report "an R-band image of the GRB990123 field was obtained with the Fred L. Whipple Observatory 1.2m telescope and CCD camera on Jan 24.547 UT. The Optical Tranisent (plus host galaxy) are easily visible and comparison with star-3 of GCN Circular 204 (R=18.5) gives an R magnitude of 20.4 for the OT+galaxy. This is consistent with the continuing t**(-1.5) decline shown by Bloom et al. (GCN 208)."
GCN notice #216
S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom, S. C. Odewahn, R. R. Gal (CIT), and D. A. Frail (NRAO) note on behalf of the Caltech-CARA-NRAO collaboration:
The lower limit to redshift of GRB 990123 (Kelson et al., IAUC, 7096), z =< 1.61, implies a luminosity distance D >= 3.73e28 cm, assuming a standard Friedmann cosmology with H_0 = 65 km/s/Mpc, Omega_0 = 0.2, and Lambda_0 = 0 (larger if Lambda_0 > 0).
Assuming the gamma-ray fluence reported by Feroci et al. (IAUC 7095), the implied isotropic gamma-ray energy of this burst would be > 2.3e54 erg, higher than most (or all) currently viable theoretical models can easily accomodate. The absolute magnitude of the optical counterpart at maximum light (Akerlof and McKay, GCN Circ. 205) would be M =< -36.5, i.e., > 2e16 L_sun, a remarkable number itself.
The apparent host galaxy of this burst, coincident with the optical transient (Odewahn et al., GCN Circ. 201 and IAUC 7094) detected on DPOSS (digital POSS-II) images calibrated with new CCD photometry in the field, has a Gunn r magnitude 21.9 +- 0.4, i.e., R ~ 21.5 +- 0.5 (cf. Gal et al., GCN Circ. 213). Its probable redshift is likely to be approximately 0.2 to 0.3, and (unless an active nucleus is present, for which there is no spectroscopic evidence so far) almost certainly not at z = 1.61, the absorption line redshift measured by Kelson et al.
We therefore suggest that this may be the first documented case of a gravitationally lensed GRB, magnified by at least an order of magnitude by a foreground galaxy detected in DPOSS (possibly including some microlensing). This hypothesis naturally accounts for all of the data so far: the large fluence of the burst, its enormous implied gamma-ray luminosity, its remarkably high peak optical brightness and implied luminosity (and makes it consistent with previous non-detections of other bursts by the ROTSE and LOTIS experiments, with m_lim ~ 14), and the presence of an apparent foreground galaxy positionally coincident with the optical transient.
The conclusive test of this hypothesis will be a direct measurement of the galaxy's redshift, once the OT fades sufficiently. If microlensing is involved, a signature may be seen in the shape of the OT light curve. Possible image splitting may be detectable, and high-resolution imaging in radio, near-IR, and optical is urged.
In any case, the early detection by the ROTSE team (GCN Citc. 205), even if corrected for a putative large gravitational lensing magnification factor, suggests that routine early detections of GRBs in the visible light are within reach, perhaps at ~ 15 mag level.
GCN notice #217
J. Zhu, J. S. Chen, and H. T. Zhang, on behalf of the Beijing Astronomical Observatory GRB team report:
"We observed the GRB 990123 OT (GCN #202) during Jan. 24.730-24.861 with BAO 0.6-m Schmidt telescope under non-photometric weather. Combination of two relatively best quality 60-min. images taken with BATC band-i filter (central wavelength at 6660 A, band width 480 A) gives a detection of the object (OT+galaxy) at RA=15:25:30.17, Dec=+44:45:59.7 (J2000) with magnitude of 21.0 +/- 0.3 (Jan 24.818) using the No. 1 and No. 2 stars in GCN #204. No visual seperation between OT and galaxy could be recognized because of low S/N ratio and bad spatial resolution (1 pixel = 1.7", seeing = 5.6"). Central 4' X 4' part of the whole 58' X 58' CCD field will soon be available under http://vega.bac.pku.edu.cn/~zj/grb/grb990123.html
Re-reduction of GCN #204 observation with only first 2 of the 4 stars gives same result for the OT+galaxy in magnitude of 19.2 +/- 0.1 (Jan 23.756).
GCN notice #218
J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, S. C. Odewahn (CIT), R. Sagar, A. K. Pandey, Neelakshi, R. K. S. Yadav (U. P. State Observatory, India) report on behalf of the Caltech-NRAO-CARA GRB collaboration:
"On 24.0 Jan 1999 UT, we imaged the optical transient (GCN #201) of GRB 990123 (GCN #199) with the 1.04-m UPSO telescope, India. In one 50-minute B-band image the transient is well-detected at B = 20.16 +/- 0.15 (Jan 23.958 UT). The transient is also well-detected in a 60-minute stacked R-band exposure at gunn-r = 20.02 +/- 0.11. The B-band image was calibrated to the zeropoint of GCN #206 with a set of secondary stars in the OT field. The R-band photometry was tied the gunn-r system using the Palomar 60-inch photometry (#207). The error associated with the calibration between the two somewhat different bandpasses is small compared to the statistical uncertainties. These measurements have not been corrected for Galactic extinction which would amount to A_B = 0.069 and A_r = 0.041 (GCN #207). With the Galactic extinction correction, the transient still blue (ie. has a positive spectral index beta ~= 0.3) at this second epoch (see GCN #207).
Assuming a power-law decay (f_nu[nu] = const*t^alpha) we find
```
    alpha_B = -0.90 +/- 0.11    (between Jan 23.58 and Jan 23.95)
    alpha_r = -1.04 +/- 0.14    (between Jan 23.56 and Jan 24.00)
```
This decay is significantly flatter than alpha = -1.5 found at earlier times (ie. t <= Jan 23.5; GCN #208). A recalibration of the transient magnitude reported by Garnavich et al. (GCN #215) using the reference star from Gal et al. (GCN #207), gives r = 21.0 (errors not reported) on Jan 24.547 UT. This implies a decay of alpha = -1.26, consistent with that found above.
An updated light-curve may be obtained at http://astro.caltech.edu/~jsb/GRB/grb990123.html
GCN notice #219
J. Hjorth (Copenhagen), M. I. Andersen (NOT), L. M. Cairos, N. Caon, M. Zapatero Osorio (IAC), H. Pedersen, B. Lindgren (Copenhagen), A. J. Castro Tirado (LAEFF, IAA), and E. Perez (IAA) report:
"Three 40 min spectra of GRB 990123 were obtained at NOT on Jan 24.24 UT 1999. Based on a preliminary reduction and analysis we detect the following ultraviolet absorption lines: Si II (152.6 nm), C IV (154.9 nm), Fe II (160.8 nm), Al II (167.1 nm), Zn II (202.6 and 206.3 nm), Fe II (234.4, 237.5, and 238.3 nm), and Fe II (258.7 and 260.0 nm) at a redshift of 1.600 +- 0.001. This value is in agreement with the redshift found by Kelson et al. (IAU Circ. 7096). Several additional absorption lines and two emission lines are detected. There is one definite system at z = 0.286 +- 0.001 based on [OII] (372.7 nm) emission and Ca H and K (393.3 nm and 396.9 nm) absorption and another possible system at z = 0.210 +- 0.002 based on [OII] emission and Ca K absorption. One of these systems are likely to be associated with the galaxy located 1.8 +- 0.4" from the optical transient (Bloom et al., GCN Circ. 206). This galaxy has an estimated redshift of z ~ 0.2 - 0.3 (Gal et al., GCN Circ. 213).
These observations support the idea that GRB 990123 may be a gravitationally lensed gamma-ray burst (Djorgovski et al. GCN Circ. 216). The 'source' (optical transient) redshift (z >= 1.60) and 'lens' (galaxy) redshift(s) (z = 0.29, z = 0.21) as well as the angular separation between the image and the lens are typical of those found among multiply imaged QSOs. The existence of several galaxies along the line of sight to the GRB enhances the lensing cross section. If GRB 990123 is multiply lensed, a fainter GRB is expected to appear 2-3" north of GRB 990123, within a time scale of months. In addition, two more GRBs may appear within days or weeks from now if GRB 990123 is quadruply lensed. Continued monitoring is urged at all wavelengths in order to determine precise time delay(s).
The results reported in this circular are preliminary. A more detailed analysis is ongoing."

GCN notice #220
A. Maury, M. Boer, S. Chaty, report:
We observed GRB 990123 from the Observatoire de la Cote d'Azur 0.9m Schmidt Telescope. The optical counterpart of the GRB is clearly detected as well as the host galaxy. Below are the unfiltered magnitudes, corresponding approximately to R magnitudes, given the detector response. The mean uncertainty is 0.4 mag. The resulting decay slope, is 1.35, which is in good agreement with the slope mentionned by Bloom et al. (GCN Report #208), given the uncertainties.

January
24.037037       19.68
24.052419       19.67
24.057234       20.02
24.061887       19.79
24.066539       19.71
24.071192       19.44
24.075845       20.4
24.080498       19.92
24.08515        20.05
24.094456       20.25
24.099109       20.15
24.113067       19.65
24.117731       19.82
24.122373       20.22
24.127025       20.53
24.131678       20.32
24.136331       20.49
24.140984       20.15
24.145637       19.98
24.150289       20.06
24.154942       20.44
24.159595       20.42
24.164248       20.03
24.1689         20.11
24.178206       20.23
24.182859       20.06
24.201076       20.36

Other images have been taken during the night 24-25/1/99 and will be reported later.

GCN notice #221
E. L. Turner (Princeton Univ. Obsv.):
Djorgovski et al. (GCN Circ. 216) suggested that GRB 990123 might be strongly lensed by an intervening galaxy based on the extreme energetics implied by its high gamma-ray fluence (Feroci et al., IAUC 7095) and the redshift lower bound of 1.61 (Kelson et al., IAUC 7096) and on the relatively bright coincident galaxy found on the digital POSS-II images by Odewahn et al. (GCN Circ. 201 and IAUC 7094). Their prediction that this galaxy's redshift would be much smaller than 1.6 and probably in the range 0.2 to 0.3 was quickly confirmed by Hjorth et al. (GCN Circ. 219) who in fact found one definite galaxian redshift system at z = 0.286 and a second possible one at z = 0.210, thus providing strong apparent support for the gravitational lensing hypothesis in GRB 990123.
Hjorth et al. also note that additional images of this GRB may appear on a time scale of days to months due to differential lensing time delays.
The light from the host galaxy of GRB 990123 will also be affected by any strong gravitational lensing which has influenced the burst and should show strongly distorted and probably multiple images if the magnification factor is large. Detection and characterization of such lensed host galaxy images would provide strong additional evidence for the lensing hypothesis but, more importantly and much more urgently, would also allow detailed modeling of the lensing mass distribution and geometry and, thus, far more precise prediction of where and when additional images of the burst are expected to occur.
These predictions would make it far more practical (and economical of observing resources) to detect and intensively study additional images of the burst as they appear. Such data would not only be valuable for probing the early stages of GRB physics but might also allow a sufficiently precise determination of the time delay(s) to use for a measurement of the cosmic distance scale. Thus, deep and high resolution imaging in search of the possibly distorted and multiple images of the GRB 990123 host galaxy is urgently indicated.
GCN notice #222
K. Hurley, on behalf of the IPN3 team, and M. Feroci, on behalf of the BeppoSAX GRBM team, report:
We have obtained a preliminary IPN3 annulus for GRB990123 by triangulation using Ulysses and the BeppoSAX GRBM. The annulus is centered at RA(2000)=163.7197 degrees, Dec(2000)= -9.4661 degrees, with a radius of 81.3372 degrees and a 3 sigma width of 2.67 arcminutes. This annulus reduces slightly both the 2' radius SAX WFC error circle (BeppoSAX MAIL 99/2) as well as the 1.5' MECS error circle (BeppoSAX MAIL 99/3), and contains the optical transient (Bloom et al., GCN 206). An image may be found at ssl.berkeley.edu/ipn3/990123/. Considerable reduction of the annulus width is possible. This report was delayed due to the fact that Ulysses data are not processed and distributed over the weekend.
GCN notice #223
Andy Fruchter reports for Dan Kelson who is in transit:
In addition to the z=1.6 metal absorption lines detected in the Keck spectrum of the OT associated with GRB 990123 and reported by Kelson et al. in IAUC 7096, H and K absorption corresponding to a redshift of z~0.2 were found in a nearby galaxy, thus confirming the redshift reported by Hjorth et al. in GCN 219. However, this galaxy is the one visible in the finding chart of Odewahn et al., GCN 201, which lies approximately 10" to the west of the OT. No evidence of lines corresponding to a z~0.2 were found in the Keck spectrum any closer on the sky.
GCN notice #224
R. M. Kippen (University of Alabama in Huntsville) reports on behalf of the BATSE GRB team:
GRB 990123 was detected by BATSE on 1998 January 23.407594 as trigger number 7343. The event was strong and consisted of a multi-peaked temporal structure lasting >100 s, with significant spectral evolution. The T50 and T90 durations are 29.82 (-/+ 0.10) s and 63.30 (-/+ 0.26) s, respectively. The burst's peak flux (50-300 keV; integrated over 1.024 s) and fluence (>20 keV) are 16.42 (-/+ 0.12) photons cmE-2 sE-1 and 5.09 (-/+ 0.02) x 10E-4 erg cmE-2, respectively---ranking it in the top 2% (0.3%) of the BATSE burst flux (fluence) distribution. The average spectral hardness of the burst, as estimated by the ratio of 100-300 keV counts to those in the 50-100 keV range, is H32 = 1.51 (-/+ 0.004), which is average among BATSE bursts. The BATSE burst location is consistent with those measured by BeppoSAX (GCN 199,202,203) and with the proposed optical/radio transient counterpart (GCN 201; 204-221). A location sky-map and lightcurve for this event (and other notable bursts) are available at the BATSE Rapid Burst Response world-wide-web site: http://www.batse.msfc.nasa.gov/~kippen/batserbr/
GCN notice #225
This GCN is an addendum to GCN 223:
Please note that Hjorth et al., GCN 219, do not indicate the position angle of the slit they used. Furthermore, to the eye, the OT appears offset to the south of the apparent host in the finding chart of Odewahn et al. (GCN 201, see also Gal et al., GCN 213), while the Keck slit lay in the E-W direction. Thus the apparent non-detection by Keck of an object reported in the NOT spectrum may be due to different position angles having been used in the observations.
GCN notice #226
J. Zhu, J. S. Chen, and H. T. Zhang, on behalf of the Beijing Astronomical Observatory GRB team report:
"We observed the GRB 990123 OT (GCN #202) around Jan. 25.9 with BAO 0.6-m Schmidt telescope under good weather. One 40-min. exposure was made with BATC band-i filter and the very weak source at RA=15:25:30.35, Dec=44:45:59.4 (J2000) was detected near the detection limit of the image which gives a magnitude of 21.3 +/- 0.3 (Jan 25.901) using the No. 1 and No. 2 stars in GCN #204. Assuming a minimum decay slope of 1.18 (for the OT) obtained from our previous observations (GCN #217), we conclude that the source we detect here is probably the coincident galaxy found on the digital POSS-II image by Odewahn et al. (GCN #201 and IAUC 7094).
GCN notice #227
R.Sagar, A.K. Pandey, R.K.S. Yadav, Nilakshi and V. Mohan of U.P. State Observatory, Manora Peak, Nainital, India report:
"We have observed the optical transient (GCN #201) of GRB 990123 (GCN #199) with the 104-cm telescope of the U.P. State Observatory, Manora Peak, Nainital, India in Johnson B and Cousin R photometric passband under good photometric sky conditions. We have calibrated the images using photometric standards located in the field of open star cluster M67. In each filter three images of 20 minutes are obtained and they are stcked to improve signal to noise ratio of the optical transient. We obtain the following magnitudes:
```
          UT date       filter      mag    err

         Jan 24.90        R        21.25   0.1
         Jan 24.99        B        22.05   0.2
```
The (B-R) colour of the OT becomes redder by about 0.2 mag compared to our observations on Jan 23, 1999 (GCN #218).These results are preliminary and more accurate results will be published later.
At the Web site http://www.rri.res.in/grb990123/ all the images taken on the nights of Jan 23 and 24 are available. We have also taken images during the night of 25/26 Jan 99 and will report the results later. Sky conditions were good for photometric observations on the nights of 24/25 Jan 99 and 25/26 Jan 99 and we have observed the M67 field for calibration purposes. Precise determination of the BVRI magnitudes of the stars as well as of the OT in the field of the GRB990123 are in progress.
GCN notice #228
T. Murakami, M. Ishida and T. Dotani, Institute of Space and Astronautical Science, and A. Yoshida and N. Kawai, Institute of Physical and Chemical Research, report:
The X-ray astronomy satellite ASCA has been observing the X-ray afterglow of GRB990123 since January 25.688 (UT). The observation started about 55 hours after the burst, but the X-ray afterglow is still bright. ASCA will continue observing the source until January 26.806 (UT). With only one forth of the dataset down-linked at the tracking station, the flux is about 1 x 10E-12 erg cmE-2 sE-1 in 2 to 10 keV assuming a power-law with the photon index of -1.6.
GCN notice #229
L.A. Antonelli, A. Di Paola, Osservatorio Astronomico di Roma, G. Gandolfi (BeppoSAX Scientific Operation Center) report on behalf of SWIRT Team and BeppoSAX Team: >BR> "On 25.16 (UT) January 1999, we imaged the Optical Transient field in the K band using the 1.1m AZT-24 telescope and the near-infrared camera/spectrometer SWIRCAM at the Campo Imperatore Astronomical Observatory (http://www.mporzio.astro.it/cimperatore/WWW/) operated by the Astronomical Observatory of Rome (O.A.R.).
The 4'X4' field of view was exposed for 3300 s reaching the limiting magnitude in the K band of 17.9 +/- 0.2 mag. No source has been found within the errorbox of the O.T. (IAUC 7094). This observation provides a more compelling upper limit in K band with respect to the previous observation in the same band (GCN #214). The behaviour of the source between the two observation, at this wavelenght, is unknown.
GCN notice #230
A. Connors and R. M. Kippen, for the CGRO-COMPTEL Rapid Burst Response collaboration; and S. Barthelmy and P. Butterworth, for BACODINE/GCN, report the following:
CGRO-COMPTEL observed GRB 990123 to be extremely bright in its ~MeV energy range. The >0.7 MeV emission rose about 18 seconds after the initial BATSE trigger at 9h 46m 56.12s U.T. and lasted about 46 seconds, for a 10 sigma detection, despite its very large zenith angle (59 degrees). An initial detection notice was sent about 10 minutes after burst onset. Improved skymaps and light-curves for the full 0.2-30 MeV range have now been posted at the CGRO-COMPTEL Rapid Burst Response web-site:
http://wwwgro.unh.edu/bursts
These are still considered preliminary as our standard energy calibration is not yet complete.
GCN notice #231
S. M. Matz (Northwestern University), G. H. Share, R. Murphy, and J. D. Kurfess (Naval Research Lab) report on behalf of the OSSE team:
The OSSE instrument on board the Compton Gamma-Ray Observatory detected strong emission from GRB 990123 in its central detectors and in its active NaI shields. While the burst was not in the detectors' field of view, it was still detected to >3 MeV in two 16 s intervals. The rise and first strong peak (at about trigger+25 s) are roughly (within 1--2 s) aligned at all energies. The low-energy (<100 keV) burst emission appears to last significantly longer than the higher energy emission. Detailed spectral analysis is underway; preliminary analysis indicates that the average spectrum of the main burst can be well described above 0.8 MeV by a photon power law with an index of about -3.
Time histories from the OSSE central detectors in different energy ranges from 50 keV to >3 MeV are posted on the OSSE burst web page ( http://www.astro.nwu.edu/astro/osse/bursts/) along with a preliminary spectrum of the burst. Also available are a plot and raw data for the high time resolution (16 ms) history of the first 60 s of the burst from the triggered OSSE shield data (>100 keV).
GCN notice #232
L.A. Antonelli, A. Di Paola, R.Speziali, Osservatorio Astronomico di Roma, Italy, and G. Gandolfi, BeppoSAX Scientific Operation Center, Italy, report on behalf of SWIRT Team and BeppoSAX Team:
"We observed the Optical Transient field of GRB 990123 in the J band using the 1.1m AZT-24 telescope and the near-infrared camera/spectrometer SWIRCAM at the Campo Imperatore Astronomical Observatory operated by the Astronomical Observatory of Rome (O.A.R.).
On 25.22 (UT) January 1999, the OT field was observed for a total exposure time of 2000 s reaching the limiting magnitude in the J band of 19.5 +/- 0.3 mag . On 26.12 (UT) January 1999 the field was re-observed for a total exposure time of 3600 s reaching the limiting magnitude in the J band of 19.3 +/- 0.3 mag. No source has been found in both observations within the errorbox of the O.T. (IAUC 7094).
These observations provide an upper limit, in the J band, to the infrared counterpart (if any) of GRB 990123 and on its behaviour.
GCN notice #233
N. Masetti, E. Palazzi, E. Pian, F. Frontera (ITESRE-CNR, Bologna), C. Bartolini, A. Guarnieri, A. Piccioni (Astronomy Dept., Univ. Bologna), G. Valentini (Teramo Astr. Obs.) and E. Costa (IAS-CNR) report:
"We have imaged the field of GRB990123 with the Bologna Astronomical Observatory 1.5m telescope equipped with BFOSC, using Johnson B and V, and Cousins R and I filters in January 24-26.
Calibrations were done referring to GCN 206 for the B image, to GCN 204 for the R image, and using the standard star BD+252534 (Taylor et al. 1989, AJ 97, 1798) for the V and I images.
In the appended table the preliminary magnitudes of the optical transient (GCN 201) are reported along with the observation dates.
Power-law fits to the R and V band data yield temporal decay indices of alpha = 1.20 +- 0.08 and alpha = 1.25 +- 0.16, respectively (errors are 1 sigma), with satisfactory chi square values. No indication is found of a flattening of the light curve due to an underlying galaxy.
Suggestion of a slight flux increase is seen on January 25 in both R and V bands, as opposed to the overall fading trend. This is however not significant.
The BVRI light curves and the radio-to-X-ray spectral energy distribution of the transient are also shown at the Web site http://tonno.tesre.bo.cnr.it/~masetti/grb990123.html Multiwavelength observations, particularly in the near-infrared and millimetric bands, are needed and urged.
```
      Date (UT)    mag    error  band
  -----------------------------------
  1998 Jan 24.122  19.36  0.20    I
           24.142  19.79  0.10    R
           24.164  19.97  0.16    V
           24.194  20.64  0.07    B
           24.216  19.92  0.10    R
           25.138  20.91  0.10    R
           25.159  20.77  0.10    R
           25.181  21.01  0.20    V   
           25.203  20.93  0.20    V
           26.154  21.77  0.16    V
           26.154  21.56  0.10    R
```
GCN notice #234
R. Rutledge and S. R. Kulkarni (CIT) note:
Djorgovski et al. (GCN #216) have suggested that GRB 990123 is lensed by a foreground galaxy identified by Odewahn et al. (GCN #201) and presumed to be at redshift 0.21 or 0.28 (Hjorth et al. GCN #219). The basis of this argument is two fold: (1) the energetics of the GRB are reduced, as lensing would provide strong amplification, and (2) the foreground galaxy, due to its placement and likely mass, must result in some amount of lensing of a background object at the position of the optical transient.
A consequence of this lensing hypothesis is image splitting. The same burst would arrive at different times, with the time difference proportional to the image separation (e.g., Turner et al. GCN #221). Motivated by these considerations we have looked into the BATSE catalog to see if there are GRBs in the general vicinity of the location of GRB 990123 (Piro et al, GCN #199) and with close to identical profile. The two profiles need not be exactly identical since microlensing combined with source expansion can lead to changes in profile.
Within a 4-sigma error radius consistent with the GRB 990123 OT transient position, we find two double-peaked GRBs. In one of these (GRB 970627, BATSE Trigger #6279), the peaks are similar in separation and peak-width ratio to GRB 990123, although the peak intensity ratio is different by about 60+/-20%. In addition, there is excess emission in GRB 990123 following the two peaks, which is not observed from GRB 970627. However, based on the characteristics of intensity profiles, it is possible that GRB 970627 and GRB 990123 are lensed images of the same GRB event.
We estimate that the chance probability of a similar profile GRB being consistent in position is about 2%, based on the identification of 8 similar GRB intensity profiles among the approximately 2000 GRBs in the BATSE catalog. If we include in this statistic GRBs with a more dissimilar intensity profile, the chance probability increases. We find 24 double-peaked GRBs (of 2000 in the BATSE Catalog) which are comparable in peak separation (15-25 seconds) but are still dissimilar to GRB 990123, resulting in a chance probability of 6.4%. A comparison between the light curves of these two GRBs is available at http://www.srl.caltech.edu/personnel/rutledge/0123/bursts.html.
If GRB 970627 is indeed a lensed image of GRB 990123, then to explain the very long time delay between the two images (1.5 years), the positional splitting must be several arcseconds in size and would most certainly require multiple lenses or a cluster.
GCN notice #235
K. Hurley, UC Berkeley Space Sciences Laboratory, on behalf of the Ulysses GRB team, and C. Kouveliotou, Universities Space Research Association, Marshall Space Flight Center, on behalf of the BATSE team, report:
GRB 970627 (=BATSE Trigger #6279), suggested as a possible gravitationally lensed counterpart to GRB 990123 (Rutledge and Kulkarni, GCN 234), was detected by the Ulysses GRB instrument and triangulated to an annulus centered at RA(2000)=153.491, Decl.(2000)=26.188, with radius 67.841 degrees and 3 sigma width 0.065 degrees. This annulus intersects the one for GRB 990123 (GCN 222) at two locations: around RA(2000)=84.055 degrees, Decl.(2000)=+08.531 degrees, and around RA(2000)=234.866 degrees, Decl.(2000)=+37.878 degrees. Neither error box, nor the annulus for GRB 970627, includes the optical transient (GCN 206). (We note that the IPN annulus and the BATSE error circle for GRB 970627 do not intersect, nor does the BATSE error circle include the optical transient, but this is not unexpected, given that the error circle is a 1 sigma location.) We conclude that GRB 970627 is not the lensed counterpart of GRB 990123.

GCN notice #236
Shude Mao (Max Planck Inst. for Astrophysics) reports:
Djorgovski et al. (GCN Circ. 216) suggested that GRB 990123 might be highly magnified (A>10) by an intervening galaxy based on a variety of arguments. A likely scenario is that the bursting source is very close to a fold caustic. As a result, we have a PAIR of highly magnified images (we call B2 and B3) plus other additional image(s).
If this scenario is correct, then from the lensing theory, three points can be immediately made:

1. There must be a fainter burst that has arrived before GRB 990123. This
   first burst (we call B1) would be offset from GRB 990123 by about ~2".
   This expectation is valid no matter what the lens potential.
   Notice that, in this scenario, all the optical and radio afterglows
   we see now are the sum of the first-arriving burst B1 and GRB 990123.
   High resolution imaging in the radio and in the optical (with HST)
   may reveal the presence of B1.

   The time interval and intensity ratio between B1
   and GRB 990123 does depend on the model. A rough estimate
   of the time delay is a few days to a month, and the intensity
   ratio is around a few to a few tens. These predictions can be made
   more precise when the astrometries and velocity dispersion of the
   foreground galaxy are better known. The gamma-ray burst data archive
   should be searched to see whether there was such a burst. A fourth
   image (B4) much fainter than GRB 990123 may also appear after about 
   a month.
 
2. The gamma-ray burst host galaxy, since it is extended, will be distorted
   into a ring or arcs if the center of the host galaxy does not have
   a significant offset from GRB 990123. Such features can be detected with
   HST imaging (see also Turner, GCN 221). This expectation is independent
   of the models.

3. The close pair of images should have roughly equal intensities.
   The time delay between these two images depends strongly
   on the magnification; a simple model shows that it can be between
   tens of seconds to a fraction of a day. This close pair therefore
   should have ALREADY left imprints on the after glow light curves.
   The close pair should have an image splitting of about 0.05" to a
   fraction of arcsecond; HST imaging will either provide a confirmation
   or rule out this scenario.
   If the time delay between the close pair can be as short as tens
   of seconds, this raises an intriguing possibility: is GRB 990123
   itself lensed? That is, GRB 990123 may be superposed by two bursts
   coming from the close pair. These two bursts have a time lag
   of about 15 seconds, producing the first and second peaks in
   the light curve. It will be very interesting to check whether 
   the gamma-ray spectrum and light curve of GRB 990123 are consistent
   with this superposition scenario. If this hypothesis is correct, then
   the close pair should have an imaging splitting of ~0.05" and
   each is magnified by a factor of about 100.

More information (including figures) can be found at http://www.mpa-garching.mpg.de/~smao/grb.html

GCN notice #237
The name of C. A. Young, duty scientist at the time of CGRO-COMPTEL's initial rapid (~10 minute) localization of GRB 990123, was inadvertently left off the author list.
The correct attribution should be as follows:
"A. Connors, R. M. Kippen and C. A. Young, for the CGRO-COMPTEL Rapid Burst Response collaboration; and S. Barthelmy and P. Butterworth, for BACODINE/GCN, report the following:"
GCN notice #238
R. J. Nemiroff (Michigan Tech.), G. F. Marani (NRC/NASA), J. T. Bonnell (USRA/NASA), J. P. Norris (NASA/GSFC), and C. A. Meegan (NASA/MSFC) report:
There is, as yet, no primary indication that GRB 990123 has undergone any type of strong gravitational lensing.
Weak lensing: Most sources at z>1.6 will be either gravitationally amplified or (more likely) de-amplified by >5% by inhomogeneities in the gravitational field between the observer and the source (see, for example, Holz et al. astro-ph/9804271).
Strong Lensing:
Galaxy lensing: There is, as yet, no primary indication that GRB 990123 is one of multiple macro-images created by an intervening galaxy lens. In comparison with QSOs, it is not unusual for single image QSOs to be found within 3 arcseconds of a low mass galaxy (e.g. Claeskens & Surdej 1998, A&A 335, 69) or for a single image QSO to have absorption lines.
As GRB 990123 was the brightest event ever detected with a measured cosmological redshift, its macrolensing probability is high relative to other GRBs and afterglows, but still, quite possibly, low in absolute terms. The foreground galaxy near GRB 990123 is intriguing but not yet defining. The "probability" now all hinges on the unknown masses and mass distributions internal to foreground galaxies.
No other BATSE triggered GRB has been found that is consistent with a lensing interpretation. Twenty-three BATSE GRBS have occurred in the past three years within a 3-sigma error contour of GRB 990123. The closest two in light curve shape were judged to be BATSE triggers 6279 and 6698. A preliminary visual inspection reveals none of them is a close light-curve match to GRB 990123.
Millilensing: There is, as yet, no primary indication that GRB 990123 has undergone significant amplification by a compact mass on the globular-cluster scale. The two main peaks in the GRB light curve do not appear to be co-added replicas of a single light curve seperated by major peak are significantly different (> 5 sigma, preliminary analysis) in BATSE channels 1 and 4. Similarly, the ratios in peak flux between the first major peak and the last peak are also significantly different (> 5 sigma, preliminary analysis).
Microlensing: There is, as yet, no primary indication that GRB 990123 has undergone significant light curve distortion due to microlensing by compact masses on the stellar mass scale. Microlensing at low optical depths would create images separated in time by only microseconds (Nemiroff 1994, ApJ 432, 478; Nemiroff 1998, ApJ 494, L173) to milliseconds (Williams & Wijers 1997, MNRAS 286, L11), generally below minimum time scale of BATSE GRB resolution (64-ms).
We encourage, however, continued attempts to recover secondary images from any GRB or afterglow, including GRB 990123.
GCN notice #239
S. R. Kulkarni (Caltech) and D. A. Frail (NRAO) report on behalf of the Caltech-NRAO-CARA gamma-ray burst collaboration the following.
"We have continued monitoring the error box of GRB 990123 (GCN #202) with the Very Large Array (VLA). All our observations were conducted in the 8.46-GHz band. At the position of the optical transient we obtain the following fluxes:
Here is a summary of all the X-band observations to date.
```
Jan 23:  <68 microJy  (GCN #200)
Jan 24:  260 microJy  (GCN #211)
Jan 26:  <78 microJy
Jan 27:  <50 microJy
Jan 28:  <50 microJy
```
Note: The upper limits are indicated by "<" and are 2-sigma.
To summarize, the radio afterglow has been detected only once, on January 24 1999. The detection was very secure being 8-sigma. The source was seen in both intermediate frequency (IF) bands and in both senses of polarization (R and L). In contrast, the average of the flux for the period January 26-28 is <32 microJy.
This high degree of variability could be due to interstellar scattering and scintillation (ISS). However, the factor of 10 variation in the flux density requires rather extreme ISS. In addition, we expect the source to be gradually rising with time. Thus the absence of the source on three successive days is quite puzzling.
Discarding the ISS hypothesis, we have two choices. First, the radio emission detected on January 24 is some kind of precursor event to the main afterglow. Typically, the radio afterglow in the 8.46-GHz band rises to the peak flux in about 10-20 days. We have little data about the behavior of radio afterglow within a few days after the burst. Thus it is difficult to accept or reject this hypothesis.
The alternative hypothesis is that the GRB is lensed (GCN #216). At the present time, there is no firm evidence for lensing. However, the lensing idea is economical in energetics. Likewise, the lensing hypothesis offers a simpler alternative to the curious phenomenon discussed above. As discussed by various people (S. Mao GCN#236, R. Narayan, pers. comm.) a robust expectation of strong lensing is that we should first see the faint image "B1" (in the terminology of GCN #236). In this framework, we identify the radio source of Jan 24th with the radio afterglow of the B1 component. If this interpretation is correct then the radio afterglow of the brighter components B2 and B3 should become visible in in the next few weeks. We do not possess sufficient astrometric accuracy to see if the reported (GCN #201) optical afterglow (which is presumably due to B1+B2) and the radio afterglow are offset on the sky. Indeed, the excellent agreement (0.5 arcsec) between the positions of the optical and the radio afterglow places a limit of a delay of 30 days between components B1 and B2+B3. Clearly, radio monitoring will refute or confirm this hypothesis.
GCN notice #240
J. S. Bloom, C. Koresko, S. R. Kulkarni, S. G. Djorgovski, R. R. Gal, and S. C. Odewahn, Caltech, H. I. Teplitz, NASA Goddard Space Flight Center, D. Koerner, Univ. of Pennsylvania, D. Kirkpatrick, Infrared Processing and Analysis Center, M. A. Malkan, and I. S. McLean, Univ. of California, LA, D. A. Frail, National Radio Astronomy Observatory, report on behalf of the Caltech-CARA-NRAO GRB collaboration:
"The field of the optical transient of GRB 990123 (GCN 201) was observed with the Near Infra-red Camera (NIRC) on the Keck I telescope on 24 January 1999 UT by Koerner and Kirkpatrick. We report the discovery of the IR counterpart of the afterglow with K = 18.3 +/- 0.03 mag (epoch 24.6356 Jan 1999 UT). The same field was observed by Malkan, Teplitz and McLean on the nights of January 27.65 and 28.55 UT, and in both instances the object was readily detected. Fitting a power law to the three magnitudes yields a power law exponent of alpha_K= -1.14 +/- 0.08.
Separately, we have carried out a program of recalibrating Gunn r and R band magnitudes reported by others via the GCN. We used those measurements which reported the magnitude of a reference star (or stars) and have put all the measurements on a common photometric system. A fit to these recalibrated magnitudes yields a power law slope, alpha_r = -1.13 +/- 0.03. Within errors, the slope of the K band data is fully consistent with that obtained from the r/R band data. We adopt a mean alpha of -1.13.
The color difference between the decaying r and K fluxes is (r-K) = 2.75 mag. Converting the magnitudes to flux this color difference corresponds to beta = -0.82, where f_nu = const * nu^beta. The ratio alpha/beta = 1.38. This suggests that the cooling frequency is higher than the central frequency of the r band (about 4x10^14 Hz) even two days after the burst. We urge continued monitoring of the optical transient in the r/R band with the view of detecting the passage of the cooling break.
In contrast to the light curves measured in the r and K bands, the light curve in the B band (calibrated on a uniform photometric system) is not well described by a power law. The origin of this discrepancy is as yet unclear. Further observations are in progress."
finding charts prepared by VSNET
GSC-based wide-field charts

USNO A1.0-based close-up (with estimated V magnitudes)
GCN notice #241
Bradley E. Schaefer (Yale):
In the last few days, the GCN has had several discussions about the possibility that GRB990123 might be lensed with a high amplification. This note is to provide balance and point out several problems with this possibility. In particular, the arguments suggest that there will be no repeat lens events on short (or long) time scales.
(1) The idea of GRB990123 lensing has weak motivation.
The motivation that GRB990123 is lensed is (A) to reduce the deduced isotropic-equivalent energy (2.3x10^54 erg) to a level that can be readily explained by models, (B) to account for the lack of previous optical flash detection by LOTIS and ROTSE [GCN #216], (C) to account for GRB970627 as a lensed image [GCN #234], and (D) to explain the large radio variability [GCN #239].
(A) To claim that GRB990123 is too energetic requires a knowledge of the burst energy budget, whereas no such answer is known. Nevertheless, within current reasonable models (e.g., collapsars, merging compact objects) the typical gamma ray energies range up to ~10^52 ergs. [This forces the lens amplification, A, to be >200 or so.] Any such argument would already require that GRB971214 [3x10^53 erg; Kulkarni et al. 1998, Nature, 393, 35] and GRB980703 [2x10^53 erg; GCN #139, GCN #143] must also be lensed with large amplitude. The likelihood of three high amplitude lenses among the 17 SAX bursts is close to zero.
(B) The optical flash luminosity is indeed large [it would appear brighter than our Sun at a distance of 1 kpc], but we have no idea of what is expected, so with A=200 the source still has M~-30.7 and this is still astounding. More to the point, no previous search would have detected an optical flash with the E_gamma/E_opt ratio for GRB990123 (i.e., V~9 [GCN #205] for a gamma ray fluence of 5.1x10^-4 erg cm^-2 [GCN #224]). For example, the highest fluence event seen by GROCSE is 1.9x10^-5 erg cm^-2 with an optical limit of 8.1 mag [H. S. Park et al. 1997, ApJ, 490, 99]. For LOTIS, the strictest limit comes from GRB970223 with a fluence of 4.8x10^-5 erg cm^-2 and an optical limit of 11.0 mag [H. S. Park et al. 1997, ApJLett, 490, L21]. So there are no missing-optical-flashes to motivate a lens suggestion.
(C) For GRB970627 to be a lensed image, it must have the same light curve, spectrum, and position as GRB990123. [Microlensing could conceivably make mild changes in the light curve or spectrum by imaging different portions of the fireball, but then the time delay between images cannot be 1.5 years without simultaneously invoking high amplitude microlensing on top of high amplitude macrolensing.] The two bursts have peak-to-peak times of 12s and 17s, have greatly different peak intensity ratios, have greatly different peak shapes, and GRB970627 lacks the late time flux prominent in GRB990123. The two bursts have greatly different hardness ratio in BATSE channels 1, 2, and 3, with H21 equal 1.37 versus 0.56 and H32 equal 5.83 versus 1.14 [ http://www.batse.msfc.nasa.gov/data/grb/catalog/flux.html]. The GRB990123 OT position is 4.23 degrees away from the IPN annulus for GRB970627 which has a 3-sigma width of 0.065 degrees [GCN #235]. Thus, GRB970627 is certainly not a lensed image of GRB990123.
(D) The radio observations of GRB990123 to date show >10X variations in flux [GCN #239], but this is not qualitatively different from the 4X variations already known from GRB scintillation [e.g., Frail et al. 1997, Nature, 389, 261]. Nevertheless, GCN #239 suggests that normal variation of a GRB cannot account for this variation, and instead propose that the radio detection is of an earlier lensed image of the same GRB. However, this alternative suggestion has exactly the same problem as what it was trying to replace, since then the earlier lensed image is required to vary by >10X. That is, lensing does not solve the posed problem. So logically, there is no motivation to invoke lensing.
(2) A GRB990123 lensing event is now extremely improbable.
Within the lensing hypothesis, for simple lenses, the time delay between images will scale as the mass of the lens, with typical delays of 250 seconds for a 10^9 solar mass lens or 7 hours for a 10^11 solar mass lens [see E. Turner et al. 1984, ApJ, 284, 1]. So for the simple case, there can be no lensed event in the future. For more complex lenses, GCN #236 points out that the time delay between the two brightest and roughly equal images will be from tens of seconds to a fraction of a day. In either case, the lack of a comparable sized lens argues strongly that there will be no more images arriving in the future and that there has been no lensing at all.
The fraction of quasars that are lensed with moderate amplification is ~10^-3. The fraction of GRBs with multiple images is <10^-3 (G. Marani 1998, Thesis, George Mason). For a GRB distance of z=1.6, the expected lensing fraction is ~2x10^-3 (D. Holz et al. 1999, ApJ, 510, 54). [A correction for amplification bias is needed for this theoretical estimate, but this will not be large due to the turn over in the LogN-LogP curve.] So, for GRBs with A>~2 we expect the lensing fraction to be ~10^-3. The lensing probability scales as A^-2. For the 17 SAX bursts, we then expect a final probability of <2x10^-6 that any burst will be amplified as much as required. This probability calculation suggests strongly that GRB990123 is not lensed and thus will not have future lensing events on short or long time scales.
(3) Beaming is a better model.
Observationally, we know that the burst emission is collimated (to allow the escape of GeV photons) and that most of burst emission is coming from very small angular beams (e.g., B. Schaefer & K. C. Walker 1998, ApJLett, 511, in press; ASTRO-PH/9810271; ASTRO-PH/9802200). Theoretically, recent models produce small cones of emission, for example S. Woosley suggests a beaming factor of 0.015 while M. Rees suggests it can be as low as 10^-4 [Rome GRB Conf.]. So we have every reason to expect significant beaming factors. This expectation will immediately lower the burst energy requirements and eliminate the motivation for a GRB990123 lens. Why invoke an extremely improbable solution with no positive evidence when everyone already knows that beaming solves the problem and must be present.
GCN notice #242
I. A. Yadigaroglu, J. P. Halpern, R. Uglesich, & J. Kemp (Columbia U.) report on behalf of the MDM Observatory GRB follow-up team:
"We imaged the field of GRB 990123 in the R band on Jan. 30.52 using the MDM Observatory 2.4m telescope. A total of 40 minutes exposure was obtained in seeing of 0.9-1.0 arcsec. The optical transient is clearly detected at magnitude R = 23.01 +/- 0.24 (referenced to the comparison star of GCN #207, assuming r - R = 0.4). An independent calibration using a Landolt standard also gives a consistent magnitude. The OT position, measured with respect to the USNO-A2.0 reference system, is (J2000) RA 15:25:30.34, Dec +44:45:59.2 with an uncertainty of 0.3 arcseconds in radius. This position is consistent with that of the original detection of the OT (GCN #206). The temporal power-law decay slope connecting our observation to the first Palomar detection is alpha_r = -1.15 +/- 0.07, consistent with all previous observations, which gave alpha_r = -1.13 +/- 0.03 (GCN #240).
Thus, the OT appears to be unresolved, at a fixed position, and following a power-law decay in time.
However, we see no object corresponding to a suggested intervening galaxy that was estimated to lie 1.8 arcsec north of the OT (GCN #206). In fact, there is no other object on our image within 5 arcsec of the OT, to a limiting magnitude of approximately R = 24. Since this galaxy was only reported to be seen marginally on one POSS II red plate with R = 21.5 +/- 0.5 (GCN #213), we conclude that it probably does not exist. If true, this eliminates one of the arguments for hypothesizing that the burst is lensed." The MDM image will be posted in the near future at http://cba.phys.columbia.edu/grb/
GCN notice #243
S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom, C. Koresko, R. R. Gal, S. C. Odewahn (Caltech), M. A. Malkan, I. S. McLean (UCLA), H. I. Teplitz (GSFC), D. Koerner (U. Penn.), D. Kirkpatrick (IPAC), and D. A. Frail (NRAO), report on behalf of the Caltech-UC-NRAO-CARA GRB collaboration:
We performed a new differential astrometry between the Palomar discovery images of the optical transient (OT) associated with GRB 990123 (GCN 201, GCN 205) and the DPOSS F (red) plate containing the field. The results indicate that the apparent foreground galaxy near the OT position (GCN 201, GCN 206, GCN 207) is offset by 2.4 arcsec to the N and 1.0 arcsec to the E of the OT, with the estimated errors of 0.13 arcsec (systematic) + 0.3 arcsec (random) in each coordinate. As we noted before, at R ~ 21.5 mag this object corresponds to approximately 2-sigma detection on the plate scan.
We detect no objects down to a limiting magnitude K ~ 23 in this location in the stack of deep K-band images obtained at the Keck-I telescope on 24 and 27 January 1999 UT (GCN 240). The implied limiting color is so blue that we can exclude even the most actively star-forming galaxies. We thus conclude that the sky survey detection was spurious. This is in a complete agreement with the findings by Yadigaroglu et al. (GCN 242) in the R band.
This leaves open the question of the identification of galaxies responsible for the absorption systems at z = 0.286 and z = 0.210 reported by Hjorth et al. (GCN 219), as well as the z = 1.60 absorption system itself (Kelson et al., IAUC 7096; and GCN 219).
Using the best-fit R-band light curve power-law slope alpha = -1.13 +/- 0.03 (GCN 208, GCN 240) normalized to the K-band detection from 24 January 1999, we estimate the maximum allowed contribution to the observed K-band light in our measurements from 27 and 28 January 1999 UT from any underlying galaxy (either the host and/or the foreground absorbers). We obtain K > 22 mag for any such objects.
However, in our best-seeing (FWHM ~ 0.5 arcsec) images, from 27 January 1999 UT, we detect a faint, galaxy-like extension to the N of the OT, with an apparent center about 0.5 arcsec from the OT itself. Its rough estimated magnitude is in the range of K ~ 22 to 23 mag, in agreement with the limits derived from the light curve analysis.
Pending an independent confirmation of this detection, this object may be either a foreground dwarf galaxy responsible for one of the absorption systems reported by Hjorth et al., or a highly luminous host of the GRB itself (possibly responsible for the absorption system at z=1.6).
While there is no clear observational evidence for a strong gravitational lensing of this burst, the possibility remains open, and the presence of foreground galaxies along the line of sight suggests that some lensing magnification must be taking place (regardless of the extraordinary apparent energetics of this burst). Forthcoming observations from the Keck and the HST should clarify the situation.

GCN notice #244
Guy Pooley, MRAO, University of Cambridge, reports the following:
Five observations of the field of GRB990123 have been made with the Ryle Telescope, Cambridge UK at 15 GHz. No significant detection was made. This is a summary:

 date       UT           S     sigma
                       -- microJy --
990125  0120-1116      160      180 
990126  0116-1140      -12      180
990129  0104-0727     -197      200
990130  0738-1122      106      420
990131  0734-1119       48      300

GCN notice #245
Steven Beckwith on behalf of STScI reports:
Owing to world-wide interest in GRB 990123, the Space Telescope Science Institute will carry out imaging observations with the HST STIS \ CCD 50CCD (clear filter), dithered, using 3 orbits of Director's Discretionary Time as a service to the community. The observations are scheduled to start Monday, 8 February at 23:06:54 UT, and finish Tuesday, 9 February, 03:21:43 UT. In the spirit of a cooperative multiwavelength attempt to better identify and understand the nature of gamma-ray bursts, the HST data will be publicly available with no proprietary period immediately upon placement into the HST Data Archive, within a day or two of the observations. When we know the exact time of placement in the Archive, we will post it.
GCN notice #246
Andrew Fruchter, on behalf of the HST GRB Collaboration, reports:
As the Director of STScI, Steven Beckwith, has announced in GCN 245, service observations of GRB 990123 by HST using the STIS CCD will be released to the community immediately upon their having been processed through the standard HST data pipeline and fully ingested by the HST Archive. However, the standard data pipeline will not combine the 6 separate dither pointings, nor do a complete removal of the hot pixels and cosmic rays.
Therefore, because of the great community interest in these observations, the HST GRB Collaboration (whose time begins again with Cycle 8) plans to fully reduce the data, and make the resulting FITS images publicly available to the community via the web within a day of the release of the data by the HST Archive (the wait may be significantly less, depending upon the timing of the release of the data from the archive). A GCN containing information on how to retrieve the fully reduced images will be sent as soon as they are available.
GCN notice #247
Xiangdong Shi and Geza Gyuk (UC, San Diego):
In GCN#211,239 S. R. Kulkarni and D. A. Frail reported radio observations of GRB990123, which show a "radio flare" on Jan. 24. GRB990123 appears otherwise to be radio-quiet as of yet (GCN#200,212,239). We suggest the possibility that this "flare" resulted from the GRB external shock running into a cloud off line-of-sight. This section of the shock was decelerated efficiently and its sychrotron frequency quickly fell into the range of radio waves. This inhomogeneity of the medium may explain the abruptness and shortness of the radio emission. The "flares" in other wavelengths were brief and might also have been buried in the main afterglow.
We speculate that the cloud may have been composed of prior ejecta from the gamma-ray burst progenitor. If there are still other nearby clouds, it will be interesting to see whether the "flares" recur.
One implication of this hypothesis, if it is true, is that the GRB990123 is not likely to be strongly beamed (>10). Its intrinsic energy output would in turn be indeed huge.
GCN notice #248
I. A. Yadigaroglu & J. P. Halpern (Columbia U.) report on behalf of the MDM Observatory GRB follow-up team:
"We imaged the field of GRB 990123 in the R band on Feb. 3.54 UT using the MDM Observatory 2.4m telescope. A total of 60 minutes exposure was obtained in seeing of 1.2 arcsec. An object is detected at magnitude R = 23.9 +/- 0.25 (referenced to the comparison star of GCN #207). However, its position is approximately 0.6 arcsec north of the previous position. The new position is (J2000) RA 15:25:30.343, Dec +44:45:59.86, whereas the position on Jan. 30.52 was (J2000) RA 15:25:30.330, Dec +44:45:59.27. These positions are measured with respect to the same set of comparison stars, and they each have a statistical uncertainty of 0.3 arcseconds in radius, but negligible systematic difference. A possible extension in a Jan. 27 K-band image 0.5 arcsec to the north of the OT was described by Djorgovski et al. (GCN #243). Our new R-band image is consistent with that report.
The optical transient has faded at an accelerated rate since our last reported observation on Jan. 30.52 (GCN #242). A continuation of the alpha_r = -1.13 decay (Bloom et al. GCN #240) would have predicted an R magnitude of 23.47 on Feb. 3.54. Since the position of the optical centroid has shifted, we conclude that our measured R = 23.9 represents an upper limit to the magnitude of both the OT and any coincident galaxy, and that we are beginning to detect either an intervening galaxy or the host galaxy of the burst. We note that the accelerated decay of the OT could be an indication that the cooling frequency has passed below the optical band, or that a jet which initially was highly collimated toward us has begun to spread. An alternative interpretation in which the initial OT has disappeared, and at the same time been replaced by a fainter lensed component, seems less likely. The new MDM image will be posted in the near future at http://cba.phys.columbia.edu/grb/
GCN notice #249
J. Hjorth (Copenhagen), M. I. Andersen (NOT), H. Pedersen (Copenhagen), M. R. Zapatero-Osorio (IAC), E. Perez (IAA) and A. J. Castro Tirado (LAEFF, IAA) report:
"We have conducted an improved analysis of the NOT spectra reported in GCN #219. We detect several absorption lines in addition to the ones found in the preliminary analysis, including Al III (185.5 and 186.3 nm). The suggested assignment of two significant absorption lines with Zn II (202.6 and 206.3 nm) is uncertain. The resulting updated redshift is 1.598 +- 0.001. The detection of an emission line associated with a system at z = 0.286 is not confirmed. This considerably reduces the significance of the proposed line system at z = 0.286. The significance of the line system at z = 0.21 is unchanged and its reality remains uncertain. These findings are consistent with the Keck results (IAUC #7096; GCN #223; GCN #235). The orientation of the slit during the observations was along the parallactic angle (roughly E-W) and covered the galaxy ~10" to the W of GRB 990123. The redshift of this galaxy is 0.278 +- 0.001 based on Ca II K and H (393.3 nm and 396.9 nm), G band (400.3 nm) and Mgb (517.4 nm)."
GCN notice #250
Arne A. Henden at USNO/USRA reports:
A single BVRI photometric observation of the field of GRB981220 was acquired with the USNO 1.0-m telescope during a January run. The resultant positions and magnitudes for stars near the center of the error box can be found on our anonymous ftp site under ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb981220.dat Additional observations of this field are planned for next week, along with the fields of GRB980703 and GRB990123. These data are being made available to assist in proper calibration of photometry.
GCN notice #251
S. G. Djorgovski, S. R. Kulkarni (CIT), G. D. Illingworth (UCSC), D. D. Kelson (DTM), J. S. Bloom, S. C. Odewahn, R. R. Gal (CIT), M. Franx (Leiden), P. van Dokkum (Groningen), D. Magee (UCSC), and D. A. Frail (NRAO) note on behalf of the Caltech-UC-CARA-NRAO collaboration:
Our re-reduction of the Keck spectrum of the optical transient associated with GRB 990123 (Kelson et al., IAUC 7096) gives the following results:
We detect 12 (13) absorption lines in the spectrum of the OT, as follows:
```
 W_obs,air  W_rest,vac   z    Line ID
  4843.74   1862.78   1.6010   Al III 
  5267.29   2026.14   1.6004   Zn II 
  5361.77   2062.23   1.6007   Cr II | blend
  5361.77   2062.66   1.6002   Zn II |
  5877.17   2260.78   1.6003   Fe II 
  6096.14   2344.21   1.6012   Fe II 
  6173.87   2373.73   1.6016   Fe II 
  6195.29   2382.76   1.6008   Fe II 
  6725.75   2586.64   1.6009   Fe II 
  6759.94   2600.18   1.6005   Fe II 
  7269.47   2796.35   1.6003   Mg II 
  7289.49   2803.53   1.6008   Mg II 
  7416.97   2852.97   1.6005   Mg I  
```
The mean redshift is 1.6004 +- 0.0005 (random) +- 0.0005 (systematic). This agrees to within the quoted error with the new determination of the absorber redshift by Hjorth et al. (GCN 249).
We note the remarkably small velocity dispersion implied by these data, less than about 60 km/s in the restframe, suggesting that the lines arise from a single subgalactic-size cloud (which of course may be a part of the host galaxy's ISM), rather than from an ansamble of clouds moving within the potential well of a normal, massive galaxy. It is also possible that the GRB host is a dwarf galaxy, in which case the object detected near the line of sight both in the K band (Djorgovski et al., GCN 243) and in the R band (Yadigaroglu and Halpern, GCN 248) may be a foreground galaxy.
No other convincing absorption systems, and no emission lines are detected in these data, in the useful wavelength range of approximately 4700 to 9000 Angstroms. We do not detect Ca II H+K absorption, nor any other common absorption lines, e.g., Na D, nor any common emission lines (e.g., [O II] 3727, H alpha, H beta, etc.) from either of the two absorption systems originally proposed by Hjorth et al. (GCN 219).
We have also measured the redshift of the galaxy approximately 10 arcsec west of the OT. From 4 relatively "clean" lines, Ca II H+K, H beta, and H alpha, we derive for its redshift z = 0.2783 +- 0.0005. From 4 blended lines, CH G-band 4300, Mg I 5173+5184, Fe I + Ca I 5267, and Na D 5893, we derive z = 0.278 +- 0.001, again in an excellent agreement with Hjorth et al. (GCN 249). No absorption or emission lines corresponding to this redshift are seen in the spectrum of the OT.

GCN notice #252
Nilakshi, R.K.S. Yadav, V. Mohan, A.K. Pandey and R.Sagar of U.P. State Observatory, Manora Peak, Nainital, India report:
We have determined the Johnson BV and Cousins RI CCD magnitudes of 18 objects in the field of GRB 990123 with the 104-cm Sampurnanand telescope of the U.P. State Observatory, Manora Peak, Nainital under good photometric sky conditions. The photometric standards present in the field of open star cluster M67 have been used for calibration. The BVRI standard magnitudes of these objects alongwith RA, DEC (J2000) and DAOPHOT photometric errors prefixed with S are given below.

------------------------------------------------------------------------
Object    RA         Dec  
      h   m    s    o  '  ''    V   SV   B-V S(B-V) V-R S(V-R) V-I S(V-I)
-------------------------------------------------------------------------
 1   15  25  27.0  44 46 23.2 14.84 0.00 0.57 0.00  0.32 0.00  0.58 0.00
 2   15  25  36.5  44 44 37.6 15.47 0.00 0.63 0.01  0.36 0.01  0.67 0.01    
 3   15  25  11.8  44 46  2.1 15.96 0.00 0.64 0.00  0.41 0.01  0.78 0.01   
 4   15  25  21.1  44 46 46.9 16.28 0.00 0.67 0.00  0.39 0.00  0.71 0.01    
 5   15  25  38.6  44 43 20.3 16.72 0.00 0.60 0.01  0.32 0.01  0.63 0.01    
 6   15  25  21.7  44 46 52.7 16.80 0.00 1.35 0.01  0.82 0.00  1.54 0.01    
 7   15  25  38.8  44 44 30.1 17.65 0.06 1.29 0.07  0.83 0.08  1.34 0.08    
 8   15  25  17.9  44 46 29.5 18.10 0.01 1.13 0.01  0.66 0.01  1.17 0.01    
 9   15  25  39.1  44 44 46.1 18.48 0.05 1.32 0.07  0.67 0.08  1.25 0.07    
10   15  25  19.6  44 47 53.0 18.66 0.01 1.45 0.02  0.86 0.01  1.64 0.01    
11   15  25  19.3  44 45 58.1 18.72 0.01 0.47 0.01  --------   0.53 0.02   
12   15  25  15.6  44 45  5.6 18.96 0.03 0.87 0.04  0.53 0.04  0.98 0.05    
13   15  25  32.7  44 44 29.9 19.01 0.01 0.53 0.02  0.37 0.01  0.69 0.02    
14   15  25  25.3  44 45 24.7 19.16 0.05 1.36 0.07  0.70 0.07  1.23 0.07    
15   15  25  16.4  44 47 28.4 19.65 0.02 1.39 0.04  0.88 0.02  1.71 0.03    
16   15  25  13.8  44 44 50.1 19.87 0.03 0.43 0.04  0.27 0.03  0.34 0.06    
17   15  25  26.6  44 43 55.3 20.14 0.03 1.06 0.06  0.80 0.03  1.23 0.04
18   15  25  27.5  44 44 43.6 20.31 0.03 0.48 0.05  0.40 0.03  0.43 0.09    
--------------------------------------------------------------------

At the Web site http://www.rri.res.in/grb990123/ the manuscript dealing with the above data are available. The title of the paper is " BVRI CCD photometric standards in the field of GRB 990123". It has been accepted for publication in the Bull. Astron. Soc. India.

GCN notice #253
Christian Veillet reports:
The optical transient (OT) associated with the gamma ray burst GRB990123 has been observed with the new wide field camera CFH12k at the prime focus of the 3.6-m CFH Telescope for three consecutive nights on 1999 Feb 5.6, 6.6 and 7.6 by C. Veillet in the course of the French GRB follow-up program at CFHT (M. Boer, CESR, C. Veillet, CFHT).
A composite image of the field around the OT has been made from eighteen 10min exposures (six per night). Using the standards published by Nilakshi et al. (GCN 252), the mean R magnitude of the OT averages to R = 23.45 (0.1) on 1999 Feb 6.6. Images of the field, with a detection limit of R=25.0 (3 sigma), can be seen at the Web site http://www.cfht.hawaii.edu/~veillet/grb990123.html. No object is detected in a radius of 5" centered on the OT.
GCN notice #254
The Space Telescope Science Institute is pleased to announce that recent observations of GRB 990123 are now available in the HST Data Archive. The program ID is 8394. The three orbits of STIS CCD imaging were done as a Director's Discretionary Program as a service to the astronomical community, in response to suggestions by Fruchter, Kulkarni, and others. The data can also be found in the archive's anonymous ftp area, ftp://archive.stsci.edu/pub/misc/grb/GRB990123/ and gzipped versions of the same fits files are also available in the compressed/ subdirectory.
GCN notice #255
Andrew Fruchter, Kailash Sahu, Harry Ferugson, Mario Livio and Mark Metzger on behalf of the larger HST GRB collaboration report:
We have reduced the HST STIS CCD clear (50CCD) images of GRB 990123 announced earlier today by the Director of STScI, Steven Beckwith, in GCN 254.
We find the optical transient to be superposed on an extended irregular galaxy. A quick photometric reduction shows that the OT has a magnitude of V=25.4 +/- 0.1 and the galaxy has V=24.3 +/- 0.15. The total integrated magnitude of the galaxy and OT is therefore V~24, which agrees with Palomar 5-m observations taken approximately 12 hours before the HST observations, and which will be reported in a succeeding GCN. No correction to the STIS magnitude has yet been made to take into account the colors of the objects, which could change the estimated magnitudes from the broad STIS clear filter by ~0.1 mag.
The faint magnitude of the OT suggests that it may be declining more rapidly than the earlier power-law would predict, as suggested by the MDM group in GCN 248.
The probable host galaxy is itself composed of several irregular pieces, extended over a region more than 1" across. The major fraction of the light in the host is to the north of the OT, thus agreeing with the probable extension reported by Djorgovski et al. in GCN 243.
A gif image as well as the reduced FITS files will be posted by 16:00 EST today (9 February 1999) on http://www.stsci.edu/~fruchter/GRB/990123 and reproduced below: The HST image of GRB 990123. The square is 3."2 on a side. East is to the left; North is up.

A wide field-of-view (25."6 on a side) image of the GRB 990123. North is up; East is to the left. The box at the center surrounds the region shown in the image above.
We welcome the use of the reduced FITS image by others; however, we ask that those who do use this image cite this GCN, or a formal publication that may succeed it (the bibliographic reference of which would be available on the above web page).
GCN notice #256
S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom, G. Neugebauer, C. Koresko (Caltech), L. Armus (IPAC), S. C. Odewahn, B. R. Oppenheimer, R. R. Gal (Caltech), N. Kobayashi (NAOJ), and D. A. Frail (NRAO), report on behalf of the Caltech-CARA-NRAO GRB collaboration:
We confirm the detection of a faint galaxy approximately 0.6 arcsec due north from the optical transient (OT) associated with GRB 990123, in the K-band images obtained with the NIRC instrument at the Keck-I 10-m telescope, on the nights of 29 January 1999 and 6, 7, and 8 February 1999 UT. The presence of this object was already suggested in the Keck K-band images obtained on 27 January 1999 UT by Malkan et al., and its K-band magnitude was estimated to be about 22 to 23 (see Djorgovski et al., GCN 243). The galaxy is clearly resolved from the OT in the images obtained on 29 January by Neugebauer and Armus; it has about an equal magnitude as the OT in the images taken on 7 and 8 February by Kulkarni and Oppenheimer, i.e., K =~ 22 +- 0.7 mag.
The K-band light curve containing the light from both objects begins to show a flattening due to the presence of this galaxy. The deviation from the power-law light curve in the K-band (Bloom et al., GCN 240), assumed to have the slope alpha = -1.15 (as measured in the r band), implies the galaxy magnitude K =~ 22.4 (+0.9, -0.4; 1-sigma), in a good agreement with the previous estimates.
We interpret this object as the most likely counterpart of the absorber at z = 1.600 (IAUC 7096, GCN 219, GCN 249, GCN 251), and the probable host galaxy of the GRB. Its observed K-band magnitude is reasonable for a normal galaxy at z = 1.6.
Analysis of the HST images of the field shows the same object (GCN 255), and further details will be reported shortly.
A Keck image of the field will be posted at: http://astro.caltech.edu/~george/grb/grb990123.html and is reproduced below:
Keck NIRC image of the field, obtained on 29 January 1999 UT bu G. Neugebauer and L. Armus. The field shown is 38 arcsec square, and it is rotated about 15 degrees relative to the normar orientation (N up, E left). The probable host galaxy, just N of the OT, is marked with an arrow.
GCN notice #257
J. P. Halpern, Y. Yadigaroglu, K. M. Leighly, & J. Kemp (Columbia U.) report on behalf of the MDM Observatory GRB follow-up team:
"We imaged the field of GRB 990123 in the R band on Feb. 14.50 UT using the MDM Observatory 2.4m telescope. A total of 100 minutes exposure was obtained in seeing of 0.9 arcsec. An object is detected at a position consistent with that of the GRB host galaxy as measured in the HST STIS image (Bloom et al. astro-ph/9902182). Its magnitude is R = 23.77 +/- 0.10 (referenced to the comparison star of GCN #207), equivalent to Gunn r = 24.10. Since this is consistent with the estimate of r = 24.0-24.2 for the host galaxy alone by Bloom et al., we conclude that the optical transient is no longer making a detectable contribution to the total light in ground-based images. We also report a revised magnitude on Feb. 3.54 of R = 23.55 +/- 0.24, after more detailed analysis of those data. Further HST observations are therefore needed to follow the unusual afterglow decay of this most energetic GRB. The latest MDM image will be posted at http://cba.phys.columbia.edu/grb/
GCN notice #260
Christian Veillet reports:
The optical transient (OT) associated with the gamma ray burst GRB990123 has been observed with the new wide field camera CFH12k at the prime focus of the 3.6-m CFH Telescope for two additional nights on 1999 Feb. 8.6 and 9.6 by C. Veillet in the course of the French GRB follow-up program at CFHT (M. Boer, CESR, C. Veillet, CFHT) with the assistance of J. Anderson (University of Victoria - Canada), as a continuation of the observations reported in GCN 253 from the three previous nights.
Using only the last three nights (image quality better than 1 arc-second), photometric measurements in B, V and R using the star #18 published by Nilakshi et al. (GCN 252) give the following results:
```
R = 23.41 (0.1)  on 1999 Feb. 8.6  - 130 mn total integration time
V = 24.05 (0.1)  on 1999 Feb. 8.6  -  50 mn total integration time
B = 24.46 (0.15) on 1999 Feb. 9.6  -  30 mn total integration time
```
The V magnitude is in good agreement with the HST estimate of the OT+galaxy magnitude (V~24 GCN 255).
As suspected from the first published image based on the first three nights (GCN 253) with the first two of poor image quality, the surrounding galaxy is now clearly resolved. A composite image in R and contour maps of the GRB area and of a nearby comparison star are available at the Web site http://www.cfht.hawaii.edu/~veillet/grb990123.html
GCN notice #261
S. C. Odewahn, J. S. Bloom, S. Djorgovski, S. R. Kulkarni, and F. Harrison (CIT) report on behalf of the Caltech-NRAO-CARA GRB Collaboration:
We have processed the HST STIS images of GRB 990123 using a variety of techniques and have deduced the following magnitudes:
```
Estimates of Gunn r magnitudes from the Median stack STIS image:
  r(OT)          = 25.25 -+0.2      systematic+accidental error
  r(OT+GAL)      = 24.16 -+0.1      accidental error only
  r(A2)          = 27.18 -+0.02     accidental error only
  r(OT+GAL+A2)   = 24.09

Estimates of Gunn r magnitudes from the drizzled STIS image: 
  r(OT)          = 25.28 -+0.2    systematic+accidental error 
  r(OT+GAL)      = 24.12 -+0.1    systematic+accidental error 
  r(A2)          = 27.23 -+0.02   systematic+accidental error         
  r(OT+GAL+A2)   = 24.06         
```
Details of the reduction, the image properties and a map defining the apertures for OT, GAL, and A2 (a discrete source 0.9" from the OT) may be found at: http://astro.caltech.edu/~sco/sco1/research/grb/grb990123/stis.html
In this website we detail the procedure for obtaining by anonymous ftp our final STIS images, a deep Keck R-band image, and the Palomar 60-inch OT discovery image discussed in GCN 201. These data may be used without any need to cite the source.
GCN notice #268
BVRI photometry in the field of GRB 990123 was performed with the USNO 1.0-m telescope in Flagstaff during the February dark run. Results of that photometry have been posted at: ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb990123.dat with a more complete description of the photometric procedures in: ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb990123.readme
Objects 1-18 in the photometry file are numbered identically to Nilakshi, et. al. (GCN 252). Other objects are either fainter stars in the same field, or else brighter stars within the 11x11 arcmin field of the CCD to be used for field identification. We have omitted the obviously extended objects 7, 9, and 16. We note some photometric differences between Nilakshi and this new photometry.
Light curve and optical spectrum from Kulkarni et al. (astro-ph/9902272)
Light curve and broad-band spectrum from Galama et al. (astro-ph/9903021)
BeppoSAX images of the X-ray afterglow [courtesy L.Piro, E.Costa (IAS/CNR), J. Heise (SRON, Utrecht), F. Frontera (ITeSRE/CNR, Bologna), L. Nicastro (IFCAI/CNR, Palermo) & BeppoSAX GRB Team)].
GCN notice #307
The U. S. Naval Observatory GRB Team (A. Henden (USRA/USNO), R. Stone, F. Vrba, C. Luginbuhl, B. Canzian, J. Munn, S. Levine, H. Guetter (USNO)) report:
We have been monitoring the Optical Transient position for GRB 990123 with the USNOFS 0.2-m FASTT system (Stone et. al. 1996, AJ 111, 1721). As this is a converted transit telescope, we were not able to begin observations until the field started to transit in darkness. However, we have single-epoch data for the following nights (yymmdd UT):
```
    990225 990302 990303 990304 990305
    990309 990310 990316 990320 990321
    990322 990323 990329 990330 990331
```
Typical limiting magnitude at R was 18.3. We saw no recurrence of the transient during this period. We also report single-epoch JHK measures (with the USNO 1.55-m telescope and infrared camera) on 990202, and J measures on 990328 and 990427. Typical limiting magnitude at J was 17.5. No recurrence was seen.
GCN notice #346
Arnon Dar reports:
Gravitational collapse of neutron stars (NS) to (di)quark stars (QS) due to phase transition or mass accretion are expected to generate highly relativistic jets plus a mildly relativistic spherical explosion (e.g., Dar 1999a,b). The jets, when pointing in our direction, produce GRBs, and afterglows that decline with time like
F(t)\sim t^{-\alpha}/[1+(t/t_0)^{\beta-\alpha}]
The spherical explosion is expected to add to it an afterglow like that of GRB 980425/SN 1998bw (Galama et al 1998; , time-dilated and redshifted by 1+z. Assuming ``a standard explosion'' = 1998bw at redshift 1+z:
F(t)\sim t^{-\alpha}/[1+(t/t_0)^{\beta-\alpha}]+ SN1998bw[1+z]
For GRB 990510, \alpha= 0.76, \beta=2.4, and t_0=1.57 days is the time when the jet begins to spread. The spherical explosion is expected to produce an up turn in the time-decline of the afterglow of GRB 990510 in the R band before the end of May, reach a maximum with R magnitude m=26.7\pm 0.3 between early and mid June, decline fastly by two additional magnitudes by mid August and then decline exponentially with a much smaller rate, with a half time (1+z)=2.62 longer than that of GRB 980425/SN 1998bw, i.e., t(half)=140 days. An identical behavior is expected from the late afterglow (t>20 days after explosion) of GRB 990123 (z=1.61, Kulkarni et al. 1999).
[The above behavior explains better the behavior of the afterglow of GRB 970228 (z=0.69; Kulkarni 1999), t_max=20 days, m(max)=25 in the R band) than a single power-law and consistent with all the other measured GRB afterglows].
HST is urged to conduct observations of GRBs 990510 and 990123 in order to test these predictions and verify/falsify the proposed NS-->QS origin of GRBs.
```
Dar, A. 1999a, A\&A, in press (astro-ph/9902017)
Dar, A. 199b astro-ph/9905315
Galama, T. J. et al. 1998, Nature 395, 670 
Kulkarni, S. R. et al 1999a, Nature, 398, 389
McKenzie, E. H. \& Schaefer, B. E. 1999 astro-ph/9904397 
Stanek, K. Z. 1999, astro-ph/9905304   
Vreeswijk, P. M. et al. 1999 GCN Circ. No. 324
```
GCN notice #354
A. Fruchter (STScI), S. Thorsett (Princeton), and E. Pian (ITESRE) report for the HST GRB collaboration:
The field of GRB 990123 was reobserved by HST with the STIS CCD in open filter (50CCD) mode during two orbits on 23 March 1999, or 59.5 days after the GRB. The total exposure time was 5040s. The optical transient was visible, but had declined by 2.1 +/- 0.2 mags from that observed on 8 February 1999 by HST (Fruchter et al. 1999, Kulkarni et al 1999), to V = 27.55 +/- 0.2 mags.
Both of the HST observations of the OT of GRB 990123 fall well below the t^{-1.09} power-law behavior of the OT during the first three days (Fruchter et al. 1999). The 8 February observation lies more than 2 mags below the continuation of such a power-law. Additionally, a power-law fit of the flux density of the OT finds between the two HST observations finds t^{-1.52 +/- 0.15}; this slope is nearly 3 sigma steeper than that found through day 3, but is also noticeably shallower than that predicted from a break in the power-law due to either our now observing the edge of a collimated outflow (Meszaros and Rees 1999) or the sideways expansion of such an outflow (Kulkarni et al. 1999, Rhoads 1999).
The HST images from both epochs will be made available in gif format at http://www.stsci.edu/~fruchter/GRB/990123. The reduced HST data from the first epoch are also available in fits format at this site. The reduced data of the second observation will be made available later, in accordance with the HST GRB Collaboration policy of making reduced data publicly available upon publication in a refereed journal. References:
```
Fruchter, A. et al., 1999, Ap. J. (Letters), 519, L13.
Kulkarni, S. et al., 1999, Nature, 398, 389.
Meszaros, P. and Rees, M., 1999, MNRAS (submitted), astro-ph/9902367.
Rhoads, J., 1999, Ap. J. (submitted), astro-ph/990399.
```
The HST images of GRB 990123. On the left is the image obtained on 8 February 1999; the image on the right was taken on 23 March 1999. Each square is 3."2 on a side. East is to the left; North is up. A wider field-of-view is found at the bottom of this page. Three orbits of HST time were used to obtain the first image; only two orbits were available for the seconod observation -- and thus the slightly higher background noise in this image.
GCN notice #712
A. Fruchter (STScI), S. Thorsett (UCSB), R. Wijers (SUNY) report for the larger HST GRB collaboration:
Public HST observations in the red (LP) STIS filter should shortly be available of the host galaxy of GRB 990123. We therefore report here on the results of late-time imaging of the host by our group using the wide-band open (50CCD) filter. The images described here can be found at http://www.stsci.edu/~fruchter/GRB , and will be submitted to a journal shortly.
The field of GRB 990123 was reobserved by HST with the STIS CCD in open filter (50CCD) mode during two orbits on 7 February 2000, or 380 days after the GRB. The optical transient was no longer visible. However we have been able to use this image to further refine our earlier photometry on the OT (Fruchter et al. 1999a,b). We now find a V magnitude for the OT of 25.4 +/- 0.1 on 8 February 1999 and V = 27.7 +/- 0.15 on 23 March 1999, implying that the late time late-time light curve falls as t^{-1.7 +/- 0.1}. This represents a break of about 0.65 from the power-law decay a couple of days after outburst, and thus is roughly consistent with the beaming model of Meszaros and Rees (1999). However, this also agrees within the errors with the power-law of the early time light curve reported by ROTSE, and this may suggest a connection between the early and late-time emission.
The GRB is found to reside near the edge of the visible stellar field of the host, and is not superposed on a strong region of star-formation.
```
References:

Akerlof, C. et al, 1999, Nature, 398 400.
Fruchter, A. et al., 1999, Ap. J. (Letters), 519, L13.
Fruchter, A. et al., 1999, GCN 354.
Meszaros, P. and Rees, M., 1998, MNRAS, 306L, 39.
```
GCN notice #715
Stephen Holland, Johan Fynbo, Bjarne Thomsen (University of Aarhus), Michael Andersen (University of Oulu), Gunnlaugur Bjornsson (University of Iceland), Jens Hjorth (University of Copenhagen), Andreas Jaunsen (University of Oslo), Priya Natarajan (Universities of Cambridge, & Yale), and Nial Tanvir (University of Hertfordshire) report:
We have obtained 8224 seconds of STIS images with the F28X50LP (long pass) aperture of the host galaxy of GRB 990123. This data was taken as part of the Survey of the Host Galaxies of Gamma-Ray Bursts (Holland et al. GCN 698) approximately 509 days after the burst. Combined images are now available at http://www.ifa.au.dk/~hst/grb_hosts/data/index.html.
Using the light curve fits of Holland et al. (2000, submitted to A&A) we predict that the optical afterglow will have R = 31.6 on 15 June 2000, and thus will not be visible in the STIS images. Therefore, we used aperture photometry to determine the AB magnitudes, in the long pass filter, of the three knots found by Holland & Hjorth (1999, A&A, 344, L67). We find the following colours for the three extended knots where CL is the AB magnitude in the STIS clear aperture, LP is the AB magnitude in the STIS long pass aperture, and beta is the corresponding spectral index, f = k*nu^beta.
```
Knot   CL     LP         CL-LP       beta
  1   28.3   28.2    +0.1 +/- 0.4    -0.4
  2   28.1   27.5    +0.6 +/- 0.4    -2.6
  3   28.0   27.5    +0.5 +/- 0.4    -2.2
```
The GRB occurred on the southeast edge of Knot 1. This knot is approximately one sigma bluer than the other two knots, and two sigma bluer than the overall colour of the galaxy (V-R = 0.43 +/- 0.18; Castro-Tirado et al., 1999, Science 283, 2069). This suggests that Knot 1 might be undergoing stronger star formation than the rest of the galaxy.
GCN notice #732
A. Fruchter (STScI) reports for the HST GRB collaboration:
It has come to our attention that the web page reported in our last circular, GCN 712, did not load properly for outside users. The GRB 990123 page that they saw was unchanged from last year.
We have corrected this problem. Readers wishing to see the light curve of the OT, or the late-time imageing of the host, can either click on the animated gif of the decline of GRB 990123 at http://www.stsci.edu/~fruchter/GRB or go directly to http://www.stsci.edu/~fruchter/GRB/990123
. We have now also included a color image of the host and the wider field about GRB 990123 created using the both the open (50CCD) images (see GCN 712) and the more recent long pass STIS data (GCN 715).
astro-ph/0605016 from 30 Apr 2006
Wei: The GRB early optical flash from internal shock: application to GRB990123, GRB041219a and GRB060111b
With the successful launch of the Swift Gamma-Ray Burst Explorer, people expected the prompt optical flash like GRB990123 would be easily detected. However the fact that early optical flash have not been detected for a number of GRBs indicates the reverse shock must be suppressed. Here we explore the possibility that the optical flash may arise from the internal shock. We find that, under certain circumstance, the optical flash of GRB990123 and GRB060111b can really be explained by the internal shock. For GRB041219a, the prompt optical emission was correlated with the gamma-ray emission, we explain this feature also in the internal shock scenario, the optical emission is the low energy extension of the gamma-ray emission, and we can restrict its redshift $z\sim 0.2$. As for GRB050904, we have shown in previous paper that the optical flash was produced by synchrotron radiation and the X-ray flare was produced by the synchrotron-self-Compton mechanism. Therefore we conclude that the early optical flash of GRBs can usually come from the internal shock. Meanwhile since the condition to produce the optical flash is not easily satisfied, so the optical flash like GRB990123 should not be common in GRBs. In addition, we also discussed the synchrotron-self-Compton effect in the internal shock model, and find that for different values of parameters, there would be soft gamma-ray (100 KeV), hard gamma-ray (10 MeV) and GeV flare accompanying the optical flash. For GRB like GRB990123, a GeV flare with fluence about 10^{-8} erg cm^{-2} s^{-1} is expected, which may be detected by the GLAST satellite.
astro-ph/0612504 from 18 Dec 2006
Panaitescu: A unified picture for the gamma-ray and prompt optical emissions of GRB 990123
The prompt optical emission of GRB 990123 was uncorrelated to the gamma-ray light-curve and exhibited temporal properties similar to those of the steeply-decaying, early X-ray emission observed by Swift at the end of many bursts. These facts suggest that the optical counterpart of GRB 990123 was the large-angle emission released during (the second pulse of) the burst. If the optical and gamma-ray emissions of GRB 990123 have, indeed, the same origin then their properties require that (i) the optical counterpart was synchrotron emission and the gamma-rays arose from inverse-Compton scatterings ("synchrotron self-Compton model"), (ii) the peak-energy of the optical-synchrotron component was at ~20 eV, and (iii) the burst emission was produced by a relativistic outflow moving at Lorentz factor > 450 and at a radius > 10^{15} cm, which is comparable to the outflow deceleration radius. Because the spectrum of GRB 990123 was optically thin above 2 keV, the magnetic field behind the shock must have decayed on a length-scale of <1% of the thickness of the shocked gas, which corresponds to 10^6-10^7 plasma skin-depths. Consistency of the optical counterpart decay rate and its spectral slope (or that of the burst, if they represent different spectral components) with the expectations for the large-angle burst emission represents the most direct test of the unifying picture proposed here for GRB 990123.
1004.2720 from 19 Apr 10
P. Mimica et al.: Multiwavelength afterglow light curves from magnetized GRB flows
We use high-resolution relativistic MHD simulations coupled with a radiative transfer code to compute multiwavelength afterglow light curves of magnetized ejecta of gamma-ray bursts interacting with a uniform circumburst medium. The aim of our study is to determine how the magnetization of the ejecta at large distance from the central engine influences the afterglow emission, and to assess whether observations can be reliably used to infer the strength of the magnetic field. We find that, for typical parameters of the ejecta, the emission from the reverse shock peaks for magnetization $\sigma_0 \sim 0.01 - 0.1$ of the flow, and that it is greatly suppressed for higher $\sigma_0$. The emission from the forward shock shows an achromatic break shortly after the end of the burst marking the onset of the self-similar evolution of the blast wave. Fitting the early afterglow of GRB 990123 and 090102 with our numerical models we infer respective magnetizations of $\sigma_0 \sim 0.01$ and $\sigma_0 \sim 0.1$ for these bursts. We argue that the lack of observed reverse shock emission from the majority of the bursts can be understood if $\sigma_0 \simmore 0.1$, since we obtain that the luminosity of the reverse shock decreases significantly for $\sigma_0 \sim 1$. For ejecta with $\sigma_0 \simmore 0.1$ our models predict that there is sufficient energy left in the magnetic field, at least during an interval of ~10 times the burst duration, to produce a substantial emission if the magnetic energy can be dissipated (for instance, due to resistive effects) and radiated away.
1004.2900 from 19 Apr 10
S. B. Cenko et al.: Afterglow Observations of Fermi-LAT Gamma-Ray Bursts and the Emerging Class of Hyper-Energetic Events
Bloom, P. Chandra, D. B. Fox, D. A. Perley, J. X. Prochaska, A. V. Filippenko, K. Glazebrook, K. M. Ivarsen, M. M. Kasliwal, S. R. Kulkarni, A. P. LaCluyze, S. Lopez, A. N. Morgan, M. Pettini, V. R. Rana We present broadband (radio, optical, and X-ray) light curves and spectra of the afterglows of four long-duration gamma-ray bursts (GRBs 090323, 090328, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on the Fermi satellite. With its wide spectral bandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large isotropic energy releases (10e54 erg). Although rare, these events are particularly important for testing GRB central-engine models. When combined with spectroscopic redshifts, our afterglow data for these four events are able to constrain jet collimation angles, the density structure of the circumburst medium, and both the true radiated energy release and the kinetic energy of the outflows. In agreement with our earlier work, we find that the relativistic energy budget of at least one of these events (GRB 090926A) exceeds the canonical value of 10e51 erg by an order of magnitude. Such energies pose a severe challenge for models in which the GRB is powered by a magnetar or neutrino-driven collapsar, but remain compatible with theoretical expectations for magneto-hydrodynamical (MHD) collapsar models. Our jet opening angles (theta) are similar to those found for pre-Fermi GRBs, but the large initial Lorentz factors (Gamma_0) inferred from the detection of GeV photons imply theta Gamma_0 ~ 70-90, values which are above those predicted in MHD models of jet acceleration. Finally, we find that these Fermi-LAT events preferentially occur in a low-density circumburst environment, and we speculate that this might result from the lower mass-loss rates of their lower-metallicity progenitor stars. Future studies of Fermi-LAT afterglows in the radio with the order-of-magnitude improvement in sensitivity offered by the EVLA should definitively establish the relativistic energy budgets of these events.
1211.1032 from 7 Nov 12
Richard Harrison et al.: Magnetization Degree of Gamma-Ray Burst Fireballs: Numerical Study
The relative strength between forward and reverse shock emission in early gamma-ray burst afterglow reflects that of magnetic energy densities in the two shock regions. We numerically show that with the current standard treatment, the fireball magnetization is underestimated by up to two orders of magnitude. This discrepancy is especially large in the sub-relativistic reverse shock regime (i.e. the thin shell and intermediate regime) where most optical flashes were detected. We provide new analytic estimates of the reverse shock emission based on a better shock approximation, which well describe numerical results in the intermediate regime. We show that the reverse shock temperature at the onset of afterglow is constant, $(\bar{\Gamma}_d-1)\sim 8\times10^{-2}$, when the dimensionless parameter $\xi_{0}$ is more than several. Our approach is applied to case studies of GRB 990123 and 090102, and we find that magnetic fields in the fireballs are even stronger than previously believed.
1507.04081 from 16 Jul 15
S. Guiriec et al.: CGRO/BATSE Data Support the New Paradigm for GRB Prompt Emission and the New L$_i^nTh$-E$_peak^nTh$ L$_i^nTh$-E$_peak^rest$ L$_i^nTh$-E$_i^nTh$ L$_i^nTh$-E$_i^rest$ relation
The paradigm for GRB prompt emission is changing. Since early in the CGRO era, the empirical Band function has been considered a good description of the keV-MeV spectra although its shape is very often inconsistent with the predictions of the pure synchrotron emission scenarios. We have recently established a new observational model analyzing data of the NASA Fermi Gamma-ray Space Telescope. In this model, GRB prompt emission is a combination of three main emission components: (i) a thermal-like component that we interpreted so far as the jet photosphere emission, (ii) a non-thermal component that we interpreted so far as synchrotron radiation, and (iii) an additional non-thermal (cutoff) power-law most likely of inverse Compton origin. In this article we reanalyze some of the bright GRBs observed with CGRO/BATSE with the new model, namely GRBs 941017, 970111 and 990123. We conclude that BATSE data are fully consistent with the recent results obtained with Fermi: some bright BATSE GRBs exhibit three separate components during the prompt phase with similar spectral parameters as these reported from Fermi data. In addition, the analysis of the BATSE GRBs with the new model results in a relation between the time-resolved energy flux of the non-thermal component and its corresponding $\nu$F$_\nu$ peak energy (i.e., F$_{i}^{nTh}$-E$_{peak,i}^{nTh}$) that has a similar index as the one initially derived from Fermi data. For GRBs with known redshift (z) this results in a possible universal relation between the luminosity of the non-thermal component and its corresponding $\nu$F$_\nu$ peak energy in the rest frame (i.e., L$_{i}^{nTh}$-E$_{peak,i}^{NT,rest}$). We estimated z for GRBs 941017 and 970111 using GRB 990123--with z=1.61--as a reference. The estimated z for GRB 941017 is typical for long GRBs and the estimated z for GRB 970111 is right in the range of the expected values for this burst.

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Jochen Greiner, last update: 16-Jul-2015 [Disclaimer]